I think that check valve has something to do with the air compressor for the fuel injection but don't remember exactly.
After posting this morning, started doing some searching on other posts. I found a good one by
@etemplet and another member. He had just had his engine out for rebuild and had a chance to study all the routings. My take-away from that was the following:
Oil Pump has three outputs. Two of them go to the crankcase, one for each cylinder. The third line goes to the compressor to provide lubrication.
The oil tank filler neck has two ports on the side. One goes to the PTO throttle body and the second goes to the bottom of the compressor (oil return). If the check valve was in place, nothing broken, it seems the oil would return from the compressor and would dribble on top of the ring and have to flow through the check valve to get back in the tank. If it built up at all it would reach the hose that goes to the PTO throttle body.
I had assumed that the throttle body was helping remove fumes from the oil tank, but I'm starting to think it's not that simple. Since my filler neck has the broken ring, it's hard for me to tell if it's liquid tight, but I think it would be. If it is, the oil from the compressor has to go through the check valve to return to the tank. If it builds up above the ring that the check valve is mounted to it would get sucked toward the PTO throttle body. It seems to me that the PTO cylinder would get a little extra lubrication by design.
For me this is just interesting theory, it's not like I won't proceed. I will definitely replace the filler neck because I hate broken parts

I plan on bench testing the system before reinstalling so that will give me a chance to test the behavior of the new filler neck with the check valve installed. I will simply connect the compressor oil supply directly to the filler neck in place of the compressor return. Then I can watch for build up.