I must have been blind, but I only recently noticed an odd-looking blue "thing" under one of the bridges of one of my three Hamilton 900's. See the photo below. I couldn't -- and still cannot -- fathom what the "thing" is.

So I looked at a second of my trio; it had a virtually identical blue thing in the same location. My third Hamilton 900 does not have this piece.

So I took the two seemingly errant 900's to my watchmaker, who assured me that the blue things were original parts of the watches. However, another knowledgable person told me that they were shims a watchmaker had installed to avoid having to properly repair the watch.

I thought it very odd that I would have two Hamilton 900's (bought from different vendors) with virtually the same blue thing in them. I just checked eBay to see whether the Hamilton 900's currently being sold have the blue thing too. One cannot tell as to the first one I found. 1st 900. But the blue thing seems to be in the three others I found. [URL=2d 900 ]2d 900[/URL] 3d 900 4th 900.

One of you surely must know that the blue things are, and why so finely finished a watch as the Hamilton 900 would have this crude part.

Hi Ethan - the "blue thing" is sometimes called a "shop screw" although I am not sure what Hamilton called it. It attaches to a cam on the underside of the bridge (you can see the edge of the blued cam just to the left of the bridge in your picture). When turned about a 1/4 turn it holds the winding/setting mechanism in the winding position. This way the movement can be removed from the case and 1) the watchmaker can wind it more easily while working on it and 2) the winding/setting parts will not create any unnecessary drag on the train when it is running on the bench (out of the case). It is blued to call attention to it as something other than a plate screw.

[In looking more carefully at my Hamilton material catalog I find listed a "shop lever"]

Jerry, thanks for solving the puzzle, but should the shop screw cam (the "blue thing") be visible when the watch is in the case and in running order, or can and should watchmakers turn it so that is hidden by the bridge?

I guess that Hamilton was not concerned about the aesthetics of it. When it is turned in so that you cannot see it, it is also engaged in the winding position and you would not be able to set the watch. This is the correct position when it is in the case.

If you look at THIS THREAD in our photo gallery you will see that most of the 12-size Hamiltons had a shop screw.