Screw for speedy 145.022

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Aloha All,
Silly question. What is the part number for the missing screw in pic? How do I order one? Or what would a watchmaker call this screw? And what size would I reference?
Thanks in advance!
Will
 
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It's a case screw, I think. Holds the movement in the case, there's another on the other side.

@Archer @ChrisN