Both of my model F water pumps have the hole plugged with what appears to be solder. I’d wager a guess that the hole was used for access to machine interior features of the pump housing, then filled in afterwards. Haven’t dove deep enough into it to try and figure it out for sure. You may be able to tap it for a pipe plug, or you can solder it closed.
Best,
PM T2
He's livin' in his own private Idaho..... I hope to go out quietly in my sleep, like my grand-dad did..... and not screaming, like the passengers in his car...