Looks like the '64 Pattern ruck pouch without the frame, not to be confused with that huge arsed thing there was as well that had a head strap on it that I suspect was never really intended for human carriage.
Wrong, look at it again, it has a foot extension out of it. It is C1 bag down to the bottom of the bag where a foot long extension goes out into the back of the wearer. I have plenty of 64 bags here, and this extension makes it a way different beast.
At first glance, based upon the obvious extension, it looked like a 64 Pattern Cargo Pack, but backwards and without shoulder straps. I've never seen anything like it. It does not look like it was modified, and people were not into having such things custom-made back then.
It is marked 1972 and trial seems to be the concensus. The theory is that the army was looking for a way to have access to main pouch without removing the valise, thus the valise was put between the pack and the frame with additional pack to support the valise. I do not have the actual bag but I think know where I can get one. I may experiment for my own curiosity. I think I will let my informat know we cannot figure out exactly what he has.