Sources:

1) From: http://swampland.com/articles/view/title:pete_carr (May 2000):
How did the Hour Glass come to be?
[Pete Carr:] Gregg and Duane asked if I would like to fly back to California with them and I accepted the offer. In a twist of fate I again joined forces with the Allmans, Sandlin and Hornsby when Bob Keller, who was playing bass for them at the time, just got up and left one day before a show at the Whisky-A-Go-Go on Sunset Blvd. They asked if I would play bass for them and I accepted. I figured if Paul McCartney played guitar first and picked up the bass out of necessity I would give it a try also. It all worked out fine at the show that night and I became a permanent part of the band.

2) From: http://swampland.com/articles/view/title:johnny_sandlin (Spring 2004):
It was later on that Mabron left and Pete came in on bass, right?
[Johnny Sandlin:] Well, when Mabron [McKinney] first left, Bob Keller came back and he played bass for awhile. I don’t remember how long, maybe for several months and then he left on very sudden notice or without any notice actually. He just didn’t show up for a gig. We were playing at the Whisky which was one of our big main shows out there (in Los Angeles). Pete had been hanging with Duane and staying with Duane. Pete was the guitar player. But he had to change over to bass for awhile. After Bob Keller left, Pete played with us until the band broke up.
Now, that clarifies something for me because two or three years ago I did an interview with Wolf [Mabron McKinney] and he had said that a lot of people had said that he was the one that left y’all quickly one night, but it was not him it was the other guy.
Yeah, it was Bob Keller. We were thinking he might have killed himself or something, just to be blunt. We lived right across the street from where that HOLLYWOOD sign is.You see it in all the movies. We could go up behind the huge letters and look out over the city and we thought he may have gone up there and jumped or something. I don’t think we actually heard from him for about six months. We didn’t know if he was alive or dead or what.
Did he ever say why he disappeared?
(Laughs) It’s a funny thing because I was sitting in my apartment one day and the phone rings and it’s Bob Keller saying, “hey man, what are you doing?” (Laughs) Well, we are still waiting for you Bob. You missed a date didn’t you? At that time I wanted him to be okay and I was angry that he had left us. I thought he was my friend and didn’t know if he was dead or what. Anyway, that’s my Bob Keller story. Things just didn’t work out with him.

3) Anathalee G. "bigann" Sandlin's post on allmanbrothersband.com (November 2, 2006, 12:28 AM):
Interesting....a little confusing but interesting. I need to write with a clarification. Pete Carr played briefly with the Menits in early '67 however he was only with the band for a short while...maybe about 6 weeks or so and then he left when the band got together with Duane and Gregg. He joined the Hourglass in late 1967 right before the gig at the Whisky at the end of November.

4)
Anathalee G. Sandlin: 'A Never-Ending Groove - Johnny Sandlin's Musical Odyssey', pages 48 & 49 (Mercer University Press, 2012):
[Jonny Sandlin:] We thought we were all set with the band, but Bob Keller was only good for a couple of months. A lot of times the Whisky would let us in early so we could work up some new material for a show. One night in January '68, we went to rehearse for our gig there the next day. Everyone showed up on schedule except for Keller. No one could find him, and we were freaking, afraid something had happened. It was during a time when lots of crazy shit was going on in Los Angeles. There was peace and love, but there was also a dark side. We went back to where we were living and called everyone who might know Bob, but we couldn't find him. We thought he'd been acting a little down, and it crossed our mind he might have taken his own life or had been injured somehow. We just didn't know.
Sometimes during that period, we'd go up to the Hollywood sign and just sit and look at the city. Right behind the sign there was a valley, and we thought maybe he'd gone up there and jumped and killed himself, or maybe he'd slipped and fallen off and was lying down there, hurt. We thought of all the things you think of when someone important in your life goes missing. We went up there to search for him with no luck. Finally, we gave up. We didn't know what happened to him, but we had a show to do the next day and no bass player.
Fortunately, Pete Carr was out there and had been hanging out with us for a while, so we asked if he knew how to play bass. He said he had never played bass, but we told him it was time to learn. We needed him the next night. He knew most of the songs already and could easily have played them on the guitar, but he hadn't played bass on any of them. We ended up spending all night going over the material until we just couldn't work any longer. Pete literally learned to play the bass overnight, and he just did fine. Before long, he had it all down and was the best bass player yet. Bob Keller wasn't with us long enough to get a nickname, but Pete, because of his young age, quickly became The Beaver, or The Beav for short.
About three months after Keller disappeared, he called me like nothing had happened, just to see how things were going. I was glad to hear from him to know he was okay, but I still was pretty pissed at him and didn't really have much to say.

[Note: on http://www.chickenonaunicycle.com/Whisky-A-Go-Go History.htm there are no Hour Glass concerts at the Whisky-A-Go-Go listed for January 1968]

5)
Galadrielle Allman: 'Please Be With Me: A Song For My Father, Duane Allman', pages 101 & 102 (Spiegel & Grau, 2014):
Mabron [McKinney] was getting into strange new interests that were becoming a distraction. He would mention seeing lights in the sky, and sought out lectures by experts on UFOs and alternate realities. He would take time at practice to explain elaborate conspiracy theories and doomsday prophecies in a calm, earnest manner that was worrying. He was less interested in playing bass, so they let him go.
Bob Keller, who had played bass in the Allman Joys, replaced Mabron but not for very long. After a couple of good months, Bob simply didn’t show up for a gig. They were pissed off for days, and then they began looking for him. They even searched behind the Hollywood sign where they hung out, thinking he might have gone up there alone and slipped, or even jumped. He disappeared without a word.
When he finally called Johnny
[Sandlin] several months later, he acted like nothing had happened, and didn’t explain. Between Mabron and Keller, a bad vibe was building around the band.
They had moved to funky apartments on Lash Lane in a bid to keep costs down and it was a definite downgrade. The first day they were there, Gregg passed police in the hallway and looked through an open door in time to see them covering a corpse with a sheet. It really shook him up.
Pete Carr, a friend of Duane and Gregg’s from Daytona, happened to be visiting when Keller took off, and he picked up the slack on bass. Pete was only about seventeen and so fresh-faced, they called him the Beaver, but he was a great guitar player. He had never played bass before, but they threw him into the deep end and he swam like a champ. He learned their songs literally overnight and jumped right in to recording their second album.