Very occasionally, I'll see a small horizontal bar over a jewel on old pocket watches. What is the function of this? My amateur guess is that it's a kind of "cap", with the "T" bar putting friction against a fitted washer. But I just don't think that makes sense. Since I only have the photo, I don't know if the bar rotates with the movement of the wheel. Thanks for any insights.
I don’t know, for certain, but I can offer an opinion. Some 19th century watches do not have the typical extended pivot on the centre wheel, onto which a friction fit cannon pinion is pressed. With most watches, the cannon pinion slips on that post when the hands are set. I suspect the subject watch is the older style. These pictures cover the earlier cannon pinion style as found on my 1883 E Howard “N” size pocket watch. These were in an article I did for the June 2007 edition of the NAWCC Bulletin. On these earlier watch movements, the centre wheel arbor is drilled through, and there is no long pivot on the centre wheel dial side, to accommodate a cannon pinion. Picture 1 shows the Howard. In the centre of the movement, note the hole in the centre wheel arbor (A), (D) a post that fits into that hole, (C), a cupped tension washer, and (B), a flat washer. (E) is the cannon pinion. Post (D) is put through the flat washer (B), then through the cupped tension washer (C), concave side up. Then the post with 2 washers is slipped into (A). The watch flipped over, and the cannon pinion (E) is staked onto the protruding post. Picture 2 shows the post with the two washers, in place. Picture 3 shows the movement flipped over, and the cannon pinion staked onto the protruding post. Friction for the cannon pinion is provided by the cupped washer on the back side of the movement. This cannon pinion design originated during the era of the key wind, key set (from the back) movement. The Howard set, using the stem and crown. I suspect the Vacheron repeater has a similar arrangement.