So, I had this half-finished roll of FP4 sitting in the fridge and decided it was time to take the Leica IIIa for a walk. Took the roll out, cut a long leader on the film, loaded it and skipped over the 13 frames I had already shot (13... that number should have been a warning!). Took a few weeks to finish the roll, as EI 100 can be challenging in the winter.
I developed it for 12 minutes in Rodinal 1+50 and the film coming out of the tank looked a bit dense. And the edge markings read... "HP5"???

Well, turns out the roll of FP4 was still happily sitting in the fridge and I had accidentally loaded, shot and developed a fresh roll of HP5 at EI 100. Of course, an FP4 cartridge is blue while HP5 is green (and clearly labelled "HP5"), but I never noticed.
Not all was lost, though. The film scanned fine, but of course rather grainy. Maybe that look fits the Barnack Leica? Most of the frames even seem printable. The Massive Dev Chart lists 9 minutes for HP5 at EI 100 in Rodinal, so I was lucky to be off by only 30%.
The actual roll of FP4 came out nicely.
