Watch movements generally come out through the front, so as you can see the head of the case screw, if it was there, overlaps with the edge of the case, thus keeping the movement from coming out.
To hold the movement in the case here, someone added small piece of steel under a near by plate screw.
All sorts of interesting "fixes" are found in old watches. Somewhat counter intuitively, many type of spare parts are more easily obtained today than in the 19th century. Perhaps a watchmaker that could make a replacement for the lost screw was not available. Or maybe the watch owner came up with this solution on their own.
Some types of fixes like this, I leave in place as part of the history of the piece. This solution wasn't very stable though, so I added a proper case screw.
More creative repairs here.