It's a so-called split stem. I'm guessing the spring is for the screw-down crown operation. I don't think I've ever seen both features together before, but it looks complete. If I'm right, the crown will have an inner thread and the tube a matching external thread. Replace the seals if you want it to be watertight. If you can compress the spring far enough, slide the two parts of the stem together sideways. If that doesn't work, you should be able to push them back together end to end, the opposite of how you pulled them apart. 

I have purchased some lume to redo the hands. Luckily I have another watch with the incorrect hands lume that I can practice on first. The colour change is also matching the same colour so I can use these hands to validate the colour is correct before I remove the lume from the Omega hands. I purchased from Nocti Lumina after seeing Marshall use them on WWR and they were great. Not only did they help me pick the perfect colour but they also very kindly gave me some different coloured lume so I can tweak the colour I have if I need, but they are confident the standard colouris going to do the job.

Welcome to the forum Michael. I hope you have a large workspace, or this phase: will be over very quickly.

Is this person still around to answer questions? It would be interesting to know what they put into the barrel. Has the watch been worn regularly in the last 20 years?

I am planning on making some homemade Etachron tools from brass rod.  This will be flattened and formed into a a flat crescent, with a slot filed into it that will act as a wrench for turning studs. Would steel be better? I could use pieces of old mainspring that I have annealed to allow them to be filed to shape. Does anyone know the dimensions of the studs?  Are they all the same? I know that there are differences between Seiko and ETA.  I only have one watch on hand with Etachron (an ETA based movement), I can measure this but I don't know if that would do for all ETA based Etachron.