You might drive yourself crazy looking for the code, but why does it matter??, you know everything about the block that you need to know. It is 4" bore(from the 3970010 casting #), it is 4-bolt main(you have verified that visually), and you know what the heads are(casting #3927185, low performance). The basic shortblock in the 70's are all the same, only slight cam and head differences, very rarely forged cranks, and even more rarely forged pistons(70-73 performance engines).
No mystery here, you have a 4-bolt 350 block, what more is needed to know??
Are you hoping you have some "rare" Corvette block, which means nothing to anyone but a Corvette restorer because the 'Vette engine is no different than a truck engine, just different valve sizes and camshaft.