This one is a bit elusive, but here’s some more information on the subject. Keep in mind that this was over 30 years ago and is an obscure variant. However, while doing some research on the M113, I’m amazed at some of the existing variants - even for engineering purposes (mineclearing and stuff).
There’s been more references to the AVLB variant of the M113, mostly acknowledgement that it existed, but nothing more.
Anyways, here’s an excerpt from the article I found. If you have the time, the article itself is interesting.
THE LIGHT FANTASTIC by Ralph Zumbro
SIDEBAR: Available M113 Variants
Now, about that AVLB. Even the folks at UDLP, (formerly FMC) didn’t know about this one, as it was created about two management generations ago, and on the other side of the continent, 1n 1968. What had happened was that in RVN, an ACAV size bridge was desperately needed. The first thing that was done, was to make a bridge that would fit on an M551 chassis, but that was still too big. Next, an Engineer field modification was cooked up and formalized.
A launching frame was mounted on a standard hull, and two other M113s were used to carry standard aluminum bridge balks. In practice, they were laid out on the ground, lifted to the tow shackles with chain hoists, and then lifted off the ground with the same hoists, which were connected to the hull’s lifting rings. Crude but effective. Next a miniaturized AVLB, hydraulics and all, was designed and built, and the photo shows that it was done and existed. The bridge weighed 1224 kg and could span a 10 meter gap, and carry 1500 kg. The best information I have says that after prototyping, a further 29 were ordered in 1969. What caused them to vanish is that the war was winding down, and they were manufactured by the U.S. Army Mobility Equipment R&D Center at Fort Belvoir. If they were ever manufactured and shipped to Southeast Asia, they are STILL THERE. They may be in storage at Belvoir.
More articles/references on the M113
United Defense - M113 Family of Vehicles also from which you can be linked to the Official Site of the United Defense M113 Vehicle Program.
M113 TRACKED ARMOURED VEHICLE SERIES