ever heard of this ?

[WOLFMANmike]

So today I'm changing the fuel pump on my 65 c10 it's a sbc I've changed a many of fuel pumps on sec before so you know that hole in the front of the block that you put a longer bolt in to hold the fuel pump rod in place ? Well I thread in a bolt and it taps out with threads still left hmmmmm what gives well after a closed look the hole it not all the way through its as if it wasn't finished ? I have never seen this before till now . Any ideas as to why it's not all the way through?