My GM engine number when was it build??

[David_Hayward (RIP)]

GM = General Motors, possibly either Flint Grey Iron Foundry or more likely The McKinnon Industries Ltd, St Catherines, Ontario (foundry)
838773 = 1938 on 216 casting
CON9 = Conveyor 9, to i.d. the casting batch should there be a problem..introduced 1929
H151 = could be August 15th 1951 but must be August 15th 1941

This would make it from possibly a '1942' Model Year engine as against a late '1941' engine. The original engine would have been assembled in Walkerville transmission plant a few weeks later, in Walkerville Transmission Plant, Walker Road, Windsor as it was by then. Engines then sat around in store and then got used up at Oshawa, without attention to any sequencing.

One thing I forgot to ask I think: what is the engine number stamped on the pad on the front of the block? This will help identify its date as well as the original purpose (car, truck, light commercial, rhd). The casting date code that you found on the head will be similar to that on the block if you can locate it, on the side near that casting number.

I am not speculating but wonder if the block was swapped for a later one at some stahe?