Hi Skip.
At first glance I was somewhat excited. But after some time that feeling passed. Why? Three things.
First. According to an article in Knife World Magazine, the M3 knife was standardized in january of 1943. Production started in march of that same year. Therefor I find it highly unlikely that any prototype would still be made in 1943. The knife was already standardized. So why create (more) experimental knives?
Second. The USMC had already a standard knife - the Ka-bar - so I don't think they were looking for a new one.
And last but not least. Thinking logically and looking at the later original M3's, the M3's were marked with a year of production, the maker and the type of knife. So if I were a manufacturor making an experimental knife I would surely put my name on it. And why put 'experimental' on it? Everybody involved with the prototypes knew it was a prototype.
All in all it feels not quite right.

Ofcourse this is only an educated guess. I could be hopelessly wrong. If I am, please correct me. :-)