Watchmaker's Callipers

I got these at a local antique sale recently. They are very nicely made external callipers which are sprung shut by the sliver of steel which is part of the main steel part. They are about 6 inches long. The scale goes from 0 to 72 and the word FRANCE is stamped on the brass scale part. I couldn't initially work out what they were for as the distance between the measuring blades at the full 72 reading is about 13.5mm. That didn't seem to be any normal metric or UK units. I calculated the distance between the blades for one unit at 0.1875mm. A Google search on old French units came up with an obsolete unit called the Truchet Point equivalent to 0.188mm. 12 Truchet points make a Ligne and 12 of them make a Pouce. I also found that French and Swiss watchmakers still use this unit. I now have several facts to bore my friends with!