That is a douzieme gauge, for jewellers and watchmakers. It is a French
measuring standard based on 12ths. It has passed into to disuse in my trade.
Here in AU I know a jeweller who still uses it and he called the 0-12 “numbers”
and so on up the scale.  The French call them “lines”. Various standard pieces
and dimensions used in construction of a ring, for instance, are made of those
thicknesses or diameters. Claws, band thicknesses, setting height etc. one
“number” is 0.188mm. It just happens that 2 through 12 times that thickness land
on some very appropriate dimensions of parts of jewellery. The use of mm has
pretty much replaced that standard but in my workshops for 40 plus years very,
very often we’ve said, a fraction over or under say 0.7mm or 1.0mm and looking
at the douzieme gauge we’d be on one of the ‘numbers’ or ‘lines’
Cheers,
Peter