I am just servicing a Parker 51 English made Aerometric and found it to have a broken ( as usual ) silver breather tube and a 1950, dated, nib. The unusual thing about it was that the nib was stamped B.P. in the bottom left hand corner. I'm guessing that this is the nib's tipping material code just like RU - PL but does anyone know what it stands for. I can't find any reference anywhere