Model number 37 at Montblanc was not a fountain pen but a ballpoint; the school pen described is therefore to be assigned to the cartridge variant of model 31, even if the addition P at the cap ring is missing — it was not engraved because piston and cartridge versions have identical caps. A blue-transparent plastic insert at the barrel end is exactly the recognition feature of the cartridge version. After thorough cleaning with clear water a converter from Pelikan or Waterman fits; today's Montblanc converters by contrast have a modern thread that may not fit. A factory overhaul is not economically worthwhile for this model (service flat fee from around 250 euros), while own repair or a parts donor are significantly cheaper ways.