safety pinion". The center wheel pinion is threaded on, rather that riveted. If the mainspring breaks and the mainspring barrel flies in the opposite of normal direction, it rapidly unscrews this pinion which in just a few turns lowers enough to disengage from the barrel. When it does, the train "down stream" is then isolated and saved from potential damage.
Note that the stem on this case is not adjustable.
This setup makes cases mostly interchangeable, but to work the stem sometimes has to go further inward, or less inward when snapped in. Cases provide a way to adjust the sleeve in and out so the motion of the stem adjusts to the particular watch.
Here we have a problem. This case is not adjustable, not at all! The sleeve, usually threaded, just sits on a cutout inside the neck of the case. It's position is fixed.
The watch is just going to have to live with the slight engagement issue.
Aside: Swiss style *positive setting" mechanisms, such as modern wristwatches have, avoid this problem by making the snapping mechanism a function of the watch movement. Cases are not as interchangeable, but no adjustment is needed either.
This watch is an Elgin grade 221, 16 size, 15 jewels, made about 1902.