Suirely, that little PIN, retaining, pin axis backsight is intended to be punched out from below as easy as pie on a No4 but a bit of a pig on the No5 because the bodies spread and step or shear it. Just punch it out! The replacements inevitably came from the carpenters shop in the form of a box of suitabl diameter small panel pins. Cut to length, smack one end to spread it, push in and tap in smacked end to hold it. Done about a zillion - as at lunchtime yesterday - and they last forever!