FIXED!

I must have done something to put both parts out of kilter.

I removed the base of the camera with the back off. Some unscientific fiddling must have freed up the mechanism, and the release started working (i.e., remaining in position). So it does work like your SQ-A. Thanks again, Dave, for the tip!

I then took removed the base of the back (the part with the serial number on). The piece of metal that engages with the projection on the darkslide isn't fixed to anything (it will fall out), and it looks to have got jammed under the pivoted arm.

I can now see why kludges in place of the missing battery holder may only work for a while. The base of the battery chamber will flex (hence the need for a solid holder to insert). So I may just run some wires to an external battery taped to the side of the camera.