Found this number stamped on the bottom of my recently rebuilt 69 400 G block. the photo is looking up at the part of the block where the starter motor mounts. It's a very clear "2531" looks like the 1 may have been added separately from the first 3 numbers. Tried a search but didn't find any info. Any idea what it is? Just curious?

 

67 Toronado. Haven't a clue either.

 

I've seen this on a few blocks. I don't know what its for either.
John

 

On my 66-7 390925E blocks:

2423, 7391, 8223, 2912, 4754, 6052, 5124, 9567, 3439.

 

any chance they are a partial match to a head or oil fill tube "UNIT #" ???