Coolant bypass port?
 

Run into a problem swapping in a hand me down 13b from a gslse. Water pump housing neck has been chewed through by rust so I'm gonna swap in my 12a water pump and housing, but it doesnt have the coolant nipple in the back like the gslse housing did. Also upon further inspection i found that the nipple on the rear housing had split down the side. Question is how important is this rear nipple and what would i loose by simply sealing it off?

It is for the thermowax for EFI. If you are going carbed you can just block them both. Or in your case, just block the one on the rear iron. There are various ways to do so. Some people make it more complicated than it needs to be. Hopefully enough of your tube is intact and can accept a short piece of hose with an object of around 3/8" shoved inside and clamped to block it off. I wouldn't mention this method if I hadn't done it successfully myself.

Lol yeah the tube was split all the way to the iron unfortunately. Thinking about threading it and shoving a bolt in there or just welding it shut. Guessing it originally ran to the throttle body. This makes me feel alot better about the engine since it came with a dellorto on it already. Thanks as always Jeff.

Find a S4 water pump housing. Tap the big hole for the automatic choke and pull the nipple out and tap for a water temp sensor. You could do the same with the cast iron GSL-SE housing also.