Because I used a battery to cause a spark on twin magnetos, with a piece of cardboard to hold the points open and the magnetism virtually disappeared in the permanent magnet. The purpose was to check and confirm which pickup was sparking when the timing was done. (Important on a twin) I had to remove the maggy and remagnetise it in the normal way. ( I have a professional magnetizer. 2 people can hardly lift it.. You usually have to remagnetise a magneto if you dismantle it or if it's been sitting or bumped around).
Just a comment to the image. There's a small non conducting button on the end of the pivot shaft the "L" shaped point moves on .It's missing. The clip sits on it. It just keeps the thing in place. won't stop it running but better its there.
To Bevan, the contacting block is made of the usual electric stuff like what older radio valve sockets were made of. It's like a canvas layered bakelite. You can buy blocks of it and cut, drill or mill it. I make insulating terminals and washers out of it regularly . Nev