Not sure I can help with that one Jeff but an interesting stamp. Personally, I have not seen that date before and although IMO it looks like a legit stamp with very prominent broach marks, it really does appear quite clean and almost a bit clinical looking.
If I recall, the casting foundry was offsite and separate from the Flint engine plant, so I would imagine it would be highly unlikely for a block to be cast, cooled, delivered and then machined/stamped in another facility on the same date. One would think that Flint would have to be completely out of blocks all together, and then send for fresh foundry cores on the said casting date in order for the machine shop to build and stamp the block on the same date. I would also think the core would still be green, and highly unlikely as suitable for machining on the same day would it not? Core shift comes to mind here.
It will be interesting to see what others have to say on this one.
