Hi everyone. A minor mystery (at least for me) has been solved.

Sometimes, Swiss watch movements post 1890 are found with the letter "M" "A" or "O" stamped into the movement. Most often if there is one of these letters it will be stamped on the barrel bridge. I have wondered what they indicated for some time.

These letters indicate that the watch was imported into France and a tax was paid based upon what the watch case was made of. I have seen these markings on WWI French aircraft clocks, pocket watches and wristwatches:

Letter French English
O = Or Gold
A = Argent Silver
M = Métal Metal (anything not gold or
silver)