Z120 Replacement Block

I live in Central Vermont. I found you folks after stumbling across the Air Force tractor that David Lory has, and striking up a conversation about that tractor with him. What a great magazine and group!! After having to tend to a leaky rear main oil seal on my grandfathers TO-20 I had never noticed, but the engine ID stamping on the lower left side of the engine block isn’t stamped Z120 like I would expect, however is stamped Z129. The engine block was cast to have the external oil filter, but wasn’t machined to use that location, the engine has the sump installed filter, so my question is: why would the engine appear to be a correct Z120, but actually have the stampings of the larger engine? I thought the Z129 engine was only used on the TO-30 tractors? I do not know the history of this tractor, my grandfather purchased it used in the early eighties. Oddly enough, the engine serial number plate was missing, and I know when he bought the tractor the engine was supposedly to have been freshly rebuilt. I was thinking that possibly at some point the engine block had been replaced. I was always told the engines had a knack to getting block cracks if they had been severely overheated, I don’t know if that is true or not. October 1, 2014.

Your TO-20 has a late production Z120 replacement block. The original Z120 block was replaced at some time. When production of the Z129 engine was begun, the tooling/pattern for the Z120 block was modified to create the pattern for the Z129 block. It was then possible to use castings made with the new Z129 tooling/pattern to also make Z120 replacement blocks.
The bosses for the side mounted oil filter and the generator bracket can be found on these replacement Z120 blocks but the blocks are bored and finished as Z120 blocks. The engine serial number plate was supposed to be removed from the old block and placed on the new block. The new block was shipped with new rivets to install the serial number plate. Obviously whoever replaced the block did not install the old serial number plate.