An unusual case for my Conni.

I hope someone can help with a Constellation case puzzle.

The case is stamped as genuine, it comes with a detachable backplate marked Constellation held by 4x angled screws and looks real with the Omega Triangle, stamped FAB SUISSE SWISS MADE ACIER INOXYDABLE (sic) 168.047

My problem is that the movement sets into the backplate with the stem passing through a captive tube Which means the movement must be fitted with the stem removed. But to lock the stem down (Or remove it again) I would need to access the back of the movement, which with the stem inserted I cannot do.

I have a Constellation movement that fits, but - It is in a similar but damaged case and I can't get it out to check its calibre.

It has the look of a catch 22-situation... Unless anybody knows better? I would be most grateful if anybody can throw some light on this puzzle

Thanks for the clarification, I see how it works, Forgive me for being thick but....
How do I get to separate (or join) the split stem when it is within the tube with no detectable movement beyond the movement set pull 1 & 2?

> As simple as that. I had guessed, but didn't dare to pull too hard in case I broke it. Thank you.