Was it when they stopped stamping the midcase with “Brevet +” ? Thank you in advance for any help rendered! Cheers!
♚ SgWatchBaron ♚
the disappearance of "BREVET+" on small crown Sub midcases (as well as on the 6538 BC midcases) happened somewhere between the serials 1.52x.xxx and 1.55x.xxx (usually with caseback I.56), so it concerns the 6536/1 only - other models had an earlier or later run. And here you find the "+" (as an abbreviation of respectively with the same meaning as the "BREVET+") on the winding crown still until approximately the end of 1956 (latest I've seen had I.57), I assume an use up of preproduced leftover stuff.
So the waiving of both patent indications was probably intended simultaneously, but took for practical reasons place with a time delay.
Hope that helps