You know what I would suggest if I was you. I would take these pics over to the USMilitariaforum and ask there. Most of the guys who write the books on US Gear, hang out over there. I guarantee if it exists, they would have the documents confirming it.

If I had to take a guess, I would think it was more a one off modification than something that was produced in quantity. But honeslty that is only a guess. I have no clue. It is neat though.

But here is the forum, I can guarantee they could answer your question.

http://www.usmilitariaforum.com/forums/