Hey guys, I’ve tried googling this to no success, I’m wondering if anyone here can help me.

Recently knocked off the back piece of glass on the viewfinder of my m6, now the scene looking through the rangefinder appears OOF. If I pull back the camera some inches I can look through the viewfinder and things look ok (in focus), so I’m guessing it’s some sort of diopter that fell off, but I bought the camera used. Is there a standard diopter adjustment that comes on the M6 and if so, is that the back piece of glass (the one right in front of the eye), on the camera? Could I just buy a new one or does this sound like a repair is needed?