NavList:

Just some quick notes since Murray Buckman already found the answer (I'd love to know how you tracked it down).

• The outer ring of the watch dial was fixed
• The next ring, displaying 5 minute intervals, was rotatable by the user
• The central clock face was geared to this ring so that it rotates 1/12th as fast

To read the watch as shown in the illustration:

• Paris time is read on the outer dial: 5 minutes to 2 o'clock.
• Notice how the minutes ring has been rotated so that the little arrow in the 60 mark points at Strasbourg.
• The minutes hand points at 17 minutes past the hour on this ring.
• The hour hand points at "past 2 o'clock" on the inner dial

Quick check: Strasbourg is at longitude 7.75° E, Paris is at 2.35° E, so the difference is 5.4°. At 4 solar minutes per degree that makes 21.6 minutes.

I then figured the use of the watch was related to the railway or the telegraph. I checked on the attached map (from https://www.wikiwand.com/en/History_of_rail_transport_in_France ) and most cities within France are on there, and those that aren't would have been soon after. Lorient stood out to me because it was just a small town, but it got connected to the rail network in 1865 because of its harbour.

So far so good, but what's the point of knowing Strasbourg time when you're in Paris, or vice versa? This had me puzzled. Murray's reference explains the watch was to be used when travelling away from Paris. The train station's clocks (and schedules!) along the route were set to Paris time, but the town's clocks were set to solar time. Using this watch you could easily adjust your watch to be on time for your appointment in town, and not miss your train afterwards.

It doesn't seem to be mechanically complicated, so I hope some were actually produced.

Luc

File: