190E leak from under or behind brake booster?

[Rocambolesque]

About 1 month or 2 ago, I replaced the master cylinder on my 190E because it was leaking. When I pulled it, I saw that the booster had oil at the bottom, so I pumped all the oil out and installed the new MC.

I bench-bled the MC but I made a mistake when filling the reservoir on the car. At first I filled it with fresh fluid and tried bleeding the brakes. It took me some time to figure out the thing with the rear reservoir so I eventually ran out of fluid. The next day, I grabbed the wrong bottle from the shelf and poured that into the reservoir. I think it was just old brake fluid in that bottle, but there might have been a little bit of engine oil. I immediately realized it and drained the reservoir dry. I couldn't get the rear compartment though. Hopefully the bad oil didn't reach the rear compartment.

After that there was still oil leaking under the booster. I thought it might have been oil I couldn't reach when I wiped the area clean that just found it's way there, so i cleaned it off. Today I found out the brake fluid level dropped a little bit and saw more oil at the bottom. Even the rubber hole plug is gone (where can I get one?). Did I wreck my new MC or is there a possible issue with the booster? If I buy another MC I want to make sure it is really the problem and not something else. Thank you.