Sorry, I left my phone in the jeep.....Missed my daughter too, she was gonna buy me lunch!
There are 2 o-rings up there. One is the big round o-ring around the main pump body, the other is a square cut one that goes in a machined spot on the gear housing, it is about 1/4 id and 3/8 od, this is the pressured oil supply for the pump. It makes a mess when you leave it out. I don't have access to QSOL any more or I could dig up a picture of it.
Make sure you fill the governor with oil, it takes a long time to fill when using engine oil pressure, long enough that parts can be damaged from lack of lube.
Use grease to hold the seal ring in place