In general, watch movements go in through the front of the cases, rest on a lip or rim, and then are held from the back by case screws. These screws go in the movement close to its outer edge, and have broad heads that thus overlap the back rim of the case so the movement can not come back out through the front.
Most watches typically have two case screws. But older ones use a design having just one screw. Opposite the screw's position there is a hole in the inside of the case rim. A pin sticking out of the side of the movement goes in that hole, holding the movement from that side.
Recently, I was sent a movement for service, and a case to put it in. The movement was one of those older styles, with the pin. And at first the case seemed fine. It is the type with the hole.
Above is an image of two cases. The case at hand is on the left. The hole is visible just to the left of the pendent. The case on the right is for comparison.
But there is a problem.
Looking closely at this image we see that there is no lip alone the inside edge for the dial-side of the movement to rest on. The inside edge is smooth. The movement does fit in this case. however, expect for the pin sticking out, it would freely pass right though and out the back! Sure, the pin would prevent this, but the single case screw only stops the movement from coming out the front. The movement, installed in this case, would remain free to tip out the back.
There is a ledge or lip around the inside, just below where the hole would be, for the dial-side plate (the lower plate or main plate) of the movement to rest.