How cool!
After reading your reply, I went back to check the label with a penlight, and I definitely think the last letter is C - it doesn't look like there was room for them to write anything after that.
I took another crack at the serial number while I had the better light there, and I am pretty sure the full stamp says "7964". I can't be 100% sure of the 7, but from what's left of the number that's the one that seems right to me. There are some marks in front of that that look like it may be the bottom half of the "A". That's not part of the stamp, though, and if I hadn't already seen serial numbers in that format I probably wouldn't guess it...there's not much left of that part.
So if I'm right about the number and I can trust the Blue Book, it sounds like you were spot on...that would place it at 1951 sometime, right?
I'm not surprised to hear about the modifications...he had a habit of tinkering with everything...he was a bit eccentric, but did he ever have some good stories. That's neat that it's still worth something too...although even if you said it wasn't, it would still be priceless to me.
Well, Jules, it's looking like you've cleared up my 20 year old mystery! Thanks again for taking the time to look at this...can't wait to share it with my sister...
~K