Hello Theo
consider FR as the same mean than ZF for franzosich zestorer
Warships seized by Italy and able to be listed receive letter FR and a number :
11 and 12 for cruisers
20 to 39 for destroyers
40 to 59 for sloops
60 to 89 mines ships
90 to 109 tugs
110 to 120 submarines
230 and after auxiliaries
example ßaliste sloop scuttled Toulon ,raised ,to Italy 1943 as FR.45, German TA.12 after capitulation of Italy (TA for torpedoboot Ausland)
many numbers were just intended as few ships were in condition to be repaired in short time
FR91 was the navy tug Datura I don't know her pennant when German manning I just know USW as under sea warfare and mmit as "marine militaire" military navy
hope that suit
michel