These things can be puzzling.
I�ve done a bit of research on this topic. My OD Green 3A in my avatar has an unmarked GPW engine too, so I also wanted the history. What I have found� Record keeping was vague at best back then.
It is unlikely that your replacement GPW engine came directly from Willys. After the war ended, Ford had little use for the manufacturing machinery, or any surplus parts. They weren�t going to build them anymore. My understanding is the surplus parts ended up in the hands of parts distributors, Willys dealers, and repair shops. Some Ford parts may have ended up back with Willys, but this is hard to verify. If this happened�and this is a big �IF�, these surplus parts would have likely been used up on the CJ-2As. If the GPW engines were ever marked by Ford, my understanding is these numbers were intentionally removed before they were distributed.
Willys manufacturing� There is certain level of consistency when it comes to the engine serial number and the Willys serial number. This is how we can tell if the engine is likely an original to the Willys. This tells us that the engines were stamped pretty close to the time that the Willys were to receive its serial number.
The reason I posted the photos above was to show you how an original serial # stamping should look. The inconsistent nature of your serial number stamping tells me that this was not done by Willys. Some jurisdictions used the engine serial # to title a vehicle. If this were true for parts of Oklahoma, it�s possible that the engine supplier stamped it to match the title information. How does your title read?
Tim
