Help identifying misc. o-ring

[saleach]

hello!

I am in the process of putting my water pump installation back together and I came across a random, tiny oring at the bottom of my bay. I took photos and color marked bolts but do not remember seeing or taking off this oring. It is not the smaller oring that belongs to the thermostat assembly. It is about 1/2" in diameter.

Any guesses?

 

[aging fleet]

O-ring, water bypass pipe? It connects to the back of the waterpump.

 

[saleach]

O-ring, water bypass pipe? It connects to the back of the waterpump.

View attachment 1339102

Thats the one I meant when i said the thermostat housing. I replaced that one with one from my kit and it was slightly bigger, maybe 3/4"

 

[aging fleet]

Thats the one I meant when i said the thermostat housing. I replaced that one with one from my kit and it was slightly bigger, maybe 3/4"

I'm stumped then, sorry.