Zenith 173? What's this screw for?

[Klassiker]

I'm currently working on this pocket watch movement for a friend. The movement is clearly marked Zenith and 173, but I can't find much information about it, so maybe it's from another manufacturer, or is know by a different Zenith calibre? I'm not having any great difficulties with the service, and I don't need to replace any parts, so an exact identification isn't essential. Nevertheless, I would like to understand what the screw in the red circle is for. Any ideas?

Movement side with mystery screw marked.

 

Dial side and keyless works

[Waggy]

Looks like there are 5 screws holding down the barrel bridge, maybe the one circled is an oversized replacement? if you remove it can you see a recess (countersink) where a smaller screw should go? But... I just found this picture (middle movement) and it seems to have a hole drilled for your screw, is yours the same under the screw?

Just found another picture here, also showing the screw the same as yous, so bang goes my theory on it being an oversized replacement screw!

 

[Klassiker]

22 hours ago, Waggy said:

is yours the same under the screw?

Yes, just a threaded hole, no countersink. The screw is a plain flat-headed screw with no chamfer or shoulder, so not made to fit anywhere else. The threaded hole is so close to the edge of the bridge, it cuts into the bevel, and goes right through, but the screw isn't long enough to reach the mainplate.

[Waggy]

Maybe its there as some kind of stop for some spring or lever under the plate?

[Klassiker]

Maybe, but for what purpose? Unless something is missing, it doesn't seem to do anything, except add to the overall thickness.

[Nucejoe]

 Shares same bridges with Zenith 106, perhaps with other caliber as well. So could the screw be for spring that holds down the sweep second arbour in place, in some other variant? 

 

[Klassiker]

10 minutes ago, Nucejoe said:

So could the screw be for spring that holds down the sweep second arbour in place, in some other variant? 

Interesting idea, but I don't believe so.

I checked on Cousins, and sure enough they had a parts list for the 173, with part number 5105 described as the "Vis de soutien du fond, sur pont de barillet" or "Back support screw, on barrel bridge" according to Google Translate. So, maybe I wasn't too far away with my "it doesn't seem to do anything, except add to the overall thickness" remark. Did they put it there to prevent the gold case-back getting squashed in?

[CYCLOPS]

I believe it is a spacer of sorts, it would keep the case-back say or anything from touching or downward pressure and altering the center wheel pivot and/or the regulator since it is also raised, just protection I guess, I have seen this on old Swiss and American movements...

[nickelsilver]

1 hour ago, CYCLOPS said:

I believe it is a spacer of sorts, it would keep the case-back say or anything from touching or downward pressure and altering the center wheel pivot and/or the regulator since it is also raised, just protection I guess, I have seen this on old Swiss and American movements...

This is it. I've seen it on other calibers (jlc in particular). Back in the day casebacks could be very thin, and this guards against influence on the movement.