I have the same issue. �1949 CJ3A with a Feb 1945 GPW-6015 engine. �The serial#s have been ground off at the water pump boss and behind the oil filter housing. It was common to grind off the original serial #s prior to installing of surpluss engines.
With a little research, the additional number that is cast into the block (I think it's on the other side from the GPW-6015) will tell you the manufacture date of your GPW. �I honestly can't remember where I found this info.
I have learned that it is not uncommon to find these GPW engines in CJs. �After WWII, there were many surplus engines available, thus they were shelved and used years later in our CJ3s.
I was a bit disappointed when I found my engine didn't match my Willys, but heck, the thing runs great. A 65 year old engine without a single leak. �Now, if I could say that about the rest of the drive train, I would have something. �It leaks at the oil pan, the transmission, the transfer case, the wheels and yokes. �I don't even know where to begin. �
Tim