It appears that the mag must have been loose for a good while and that only the points side of the mag base had worn, possibly more than 1mm and there is a little bit of wear to the mag platform on the crankcases, but I'm sure a shim would sort this out. The mag was sitting with the shaft pointing upwards and making the seal housing come loose too it was that bad that when it was clamped down tight it wouldn't even allow the mag pinion to fit through the hole in the casting!
You can see how the shaft sits to the top of the casing, this was photographed when the mag wasn't clamped down, it was a lot worse when it was clamped down. So I guess the easiest/quickest way to sort it is with a mag body!
email (option): davmax@ntlworld.com
Kit,Dave. If you're not worried about originality? I have this spare post war mag with HT lead at front. It'll still work fine on an M20. I can send it as is, or strip it and just send the body. Just the cost of post is all I need. Ron
email (option): ronpier@talk21.com