Have what I believe to be a 1960's SM - 500 series top-loader, but none of my tricks have been effective in getting the movement free of the case. Can see the recesses at 5 and 11. My expectation had been that with the crystal & bezel removed that the movement & dial could be rotated 15 deg, or so, CW in the case, then getting the movement case clamps to align with the recesses. The dial & movement can be rotated very slightly (about 1 deg, maybe less), but after that nothing more. Turning the watch over does not allow the movement to drop out, although if inverted the dial will separate from the case at 9 very slightly. The keyless works side of the split stem is in the winding position, with the male coupling end level to the plane of the movement. Wrong assumption on my part about needing to rotate the movement in order to then be able to lift it out? (really seems like it is in not much of a mood to rotate anywhere...) Hope everyone is having a great weekend.
The clip ring is the correct answer, thanks @redpcar! Had not encountered this case system before, but now I know Cal 562, as it turns out
