Mystery Part from Stihl Chainsaw

I opened the case of my Sthil MS311 chainsaw and found a recessed screw in the case, but I cannot find where it came from (saw wrench head for scale). It seems to be a something used to hold liquid, because of the rubber gasket. The saw ran fine for a long time (about a year). It’s been acting up lately, not sure if this is the cause. By acting up, I mean it starts fine, but it seems like the chain isn’t getting enough bar oil, causing the chain to bind up.

I searched though the parts list on the usual sites, but cannot find it.

Any idea what it is and where it goes?