These refills are for the BIRO Minor model.
Initially released in 1948 in a capless version, Biro upgraded the pen in November 1948 to include a cap and clip
The new refill simply pushes into the pen and fits into the end of the section.
A choice of ink colours are available in all the colours Biro made, with the pen barrels and refill tips matching
Total Length132mm Colour matched refills have a 1.2mm ball and the capless has a 1mm ball.