Brian, I'm not sure what it is. It's probably proprietary. It's not a WR-75, that's the square/recatangular feed that's often found on single polarity LNBs. This has the exact dimensions of a C-120, same round shape, size, and everything, but the bolt holes are in a slightly different pattern. Probably some proprietary design from the BUC manufacturer (Gilat)
...
When my friend and I first pulled this dish from the top of a closed post office building and took it home, I told him later that day to mount one of my Hughes C-120 LNBs on the feedhorn. He called me and told me it wouldn't line up. I said "Of course it will!"... He said "I bet you it won't!" I'm glad I didn't take the bet
... We managed to get two bolt holes on opposite sides of the feedhorn to line up, and bolted the new LNB on, hoping the O-ring was enough to seal any gap that might have been left due to the omitted screws.