36 exposures has always been the max. load for regular 35mm cameras.
Back in the hey-day of the III-f (1950 - 1956), generally, pre-loaded cassettes were available either in 20 or 36 exposure rolls.
The 24 exposure 35mm roll is a fairly recent (post-1960?) development.
I've probably shot about 4 dozen rolls through my various Barnacks, but only two of them have been 36 exp.: One was Ektachrome in my 1934 III (chrome), the other (still in my 1933 black III) is Fujicolor 1800. No problems with blank frames from the Ektachrome. Have to finish the 1800 and see what that provides.
My local sources usually stock only 24 exp rolls. so that's what I've been using.
Anthony, if the blank frames were the last ones shot with the III-f, have you checked the shutter operation since then ?
( There were 250-shot versions of the Screwmount Leica, as well as the half-frame 72; the film advance /shutter cock only "cares" about advancing one frame, then stopping automatically, for proper image spacing. The limit on the total number of frames that can be shot should be limited only by the capacity of the magazine / take-up spool... Usually, I feel the advance-knob "cinch-up tight" before I reach a full advance to that 25th or 37th frame...)
That stinks that you lost some "money shots"...
My only issues with my Barnacks have been: a film jam (careless leader trimming on my part); forgetting to remove the lens cap a couple of times; forgetting to extend and lock the collapsible Elmar or Summicron ( ! ); shooting with gloves on and dragging the shutter dial as I pressed the shutter release; crackly & leaky curtains in my III-f (s/n 600xxxx).
Beyond that, mine have been very reliable; none have been CLA'd (but probably all SHOULD be...)
Luddite Frank