Question about 138ss movement

Hi,

Does anyone know what the role of the screw highlighted in the photo is?


I'm looking at potentially buying a bumper Polerouter but I noticed the movement is missing this screw, obviously without seeing the watch in person it's hard to tell what could be missing but I was hoping to get an idea of the extent of missing parts/damage caused by this missing screw.

Here's a photo of the movement of the watch in question.


As a more general question, where can I find technical info on these movements?

Cheers

This might help the discussion.

https://www.dropbox.com/s/pfbu56mvk5bqhdx/1629_Universal 138.pdf?dl=0

Its a 3rd screw that holds down that plate - the two either side of "GENEVE" are short, the missing one is longer and secures it to the main plate. Here is the screw that is missing:

Easily taken care of-

Thanks for the help, I managed to find the right screw on ebay. It should arrive in a few days so I'll post an update then.