Sid - Did you take the engine number off of the plate on the engine or from the maker's plate on the front of the tool box? The number that you qouted, 21200, is too high to be from a 51 TD, so I am assuming that it came off of the plate on the engine. If you look closely at the center of the plate, I believe that you will find that the XPAGTD portion actually reads XPAGTD2 (see attached photo) . This would be a late engine (somewhere around late 52 to 53) and will have the longer rocker shaft and the larger, 8" clutch. It will also have the later metalurgy used for the crank, which makes it much less apt to crack. Also, Tony is correct, the engine is not a Gold Seal engine, but being a later engine in the series, it is a better engine than the original, in my opinion.