2004 Yamaha Beartracker 250 gearcase plug oil leak

[masons2009]

Hey everyone I cant seem to find any answers to my problem, hoping for some help. I have a 2004 Yamaha Beartracker 250. I have an issue where my plug (don’t know name) on the middle top back of gear case where the transmission halves merge was leaking. I was actually able to push it down but it comes back up from presssure when engine cranked. The snap ring that holds the plug doesnt look like original but i put an O ring under it to hold pressure and it literally blew the whole plug off from pressure. My question is: is this plug suppose to have a lot of pressure under it like this? And what is the purpose of this plug and what is directly under it to cause so much pressure? The manual or any parts diagram doesnt acknowledge this plug or the snap ring holding it so im real puzzled at all this. I can see oil when plug came out and the oil level slowly goes back down into engine when not running. My PVC valve is hooked up and patent as should be and my gear box vent and hose is hooked up and just freely open to air. No oil comes out of any of the other hoses and this oil leak/ pressure problem has happened with low oil and when engine full of oil. Any help with this is greatly appreciated! Thanks guys!  Pictures of plug attached.