Last time I used my Leica IIIf was last summer, when I had a problem where the first roll was OK, but the first half of the second roll had frames only exposed for a third of their length, though the second half of the second roll was OK.
So as last Friday was relatively decent weather, I took it out for a test roll using all the speeds from a thirtieth up and making notes so if any of them were only part exposed I could tell which speeds were at fault.
But all the frames were OK. Which doesn't solve the mystery from last time but suggests I don't need it serviced just yet.