The ID pad can say most anything after 50 years. Could be a rebuilder stamp or something like that. If you have a casting number from the side you can at least figure out when the block was cast. If there is a VIN stamp that might help narrow it down to. A lot of 413 engines ended up in motor homes and trucks so that is probably what it is but you won't know until you look at the casting date. I think the motor home 413 engines were cast all the way into the 70's even though they stopped using them in cars long before that.