I have a couple of these movement holders for 218 Accutrons.  In order to mount the movement, the holder must be splayed ever so slightly and then released when the movement is in place.  I have been splaying with a screwdriver, but this morning, I made an add-on tool that does the work for me. I turned a slight conical profile with the initial diameter equal to the hole in the holder in rest position.  I turned to a larger diameter enough to open the holder enough for the movement to drop