Extra water jacket hole in head??

[davem201m38]

Hi al,
I wish I had a photo but the engine is 'at the shop'.

My L134 cyclinder head has an extra 1/2 inch hole in the verticle back of the head. (Not on the top of the head) It looks like it might once of been threaded but for the last 40 ish years seems to have been plugged with wood dowl !!

Has anybody else found or have the extra hole into the water jacket on the verticle rear of the head? Does it have a use?

Thanks, Dave.