Originally Posted by Speeding Big Twin
If it's authentic the belly number you found indicates the R-H case is a 1951 model Harley case. The first 1 is a code number and for 1951 it was used for 61ci Panheads and 74ci Panheads. The 51 indicates 1951 model. The remaining four numbers represent sequential machining production (numbers there began at 1000 for production models). The R-H case may have originally been mated with a Harley left case and the two halves would have been machined together as a matching pair at the factory and given the same belly number as each other. What happened to the original Harley left case I do not know. Anything could have happened to it. Anyway it seems you now have a Harley R-H case bolted to an MG left case.
Is there anything stamped on top of the R-H case near the rear engine mounting bolt? I can’t see anything there in your photos. If the R-H case was originally used for a 61ci engine then I’d expect nothing there but if it was originally used for a 74ci engine I’d expect a 7 stamped there.
Re the numbers you found on the heads, I imagine they were on the R-H side near the top and near the recesses for the pushrod tubes? If so, I think 6 77 indicates the heads were cast in June 1977.
I do not see any numbers near the rear engine mounting bolt on the RH case.
So I am assuming now this was originally 61ci.? And I did verify that the heads are 1977 shovel.