I know Nova RC used to make an aluminum differential cup for Ofna Hyper 7-8 diff's. But i dont see where it has a hole in the cup for a longer pin and set screw...I'll try and do some more digging...
I have a bag of some extra spider diff parts I was looking through yesterday. I got them with a tore up Spider diff and I have that same cap. Looks identical to yours. So I bet it is a Hyper 8.