Oh yeah, it is also rumored a few block slipped without being stampped, but now-a-days, that just means someone found a matching non k-code block and claimed it was one that wasn't stamped. Don't fall for it.
Also, the tranmission should have a vin stamp too. From pics I have seen, they are on the bottom.
I think '67 and later, all blocks were stamped. Before, just hipos.