It's unlikely that the screen is in upside down. The off-center location of the tab should prevent that. But something that consistently happens in older cameras is that through time the stop for the mirror's down position gets hammered downward very slightly by thousands of tiny returning-mirror blows. This moves the mirror farther from both lens and focus screen, increasing the distance between them. To compensate for the increased distance you need to rack the lens back accordingly, which causes back-focusing on the film when the mirror is out of the way. Look under the mirror for most cameras (I don't know about this one specifically) and you will usually see a rest for the mirror, usually on the right side as you look in.
These usually have some hint of an adjustment mechanism--a lever with a screw, an eccentric screw, something like that. My Nikon FGs have quite a complex lever and screw adjustment, but the simpler FG20 has a small post that you can easily bend up into the proper position. When you find what needs to be done, you really do NOT need to do much. So little that you think you have done nothing at all will likely be too much. Not having complex equipment to measure this, I've always relied on making visual infinity focus through the finder coincide with when the scale on the lend reads infinity, and this has not failed me on several different camera brands. I bet the way yours is right now that you can't quite get there, because the lens won't go past infinity, of course (but in the situations you have noticed, it will go past, for instance 15 feet to 17 feet). If you have a rangefinder viewing screen notice it will be even easier to see the system stops before infinity is achieved. . .
