In his last post, Magnetoman said something very important, albeit in parentheses:
Originally Posted by Magnetoman
... a magneto needs to be magnetized when fully assembled (or with proper keepers).

The Lucas N1, KN1, K1F, K2F and KVF magnetos and MO1 and MN2 magdynos include internal keepers (the extensions of the pole laminations that extend underneath the armature and almost meet opposite the magnet). That is presumably why Lucas explain in their 1953 workshop instructions specific to these models of magneto that it is not necessary to apply a keeper when removing the armature. Equally, an extra keeper is not required when removing an internal remagnetising core from and inserting the armature into these particular models.

However, some other models of Lucas magneto (such as the GJ4 I am currently working on) and many other makes of magneto (such as the Bosch discussed in this series) do not include an internal keeper. Their magnets will lose a significant amount of strength the first time they are made keeper-less. That is presumably why the Lucas workshop instructions on remagnetisation of magnetos generally, which cover a vast range of Lucas models including those without an internal keeper, make the point that sometimes it is necessary to remagnetise after an armature has been removed.

It is therefore important, when remagnetising magnetos without an internal keeper using an internal remagnetiser, to apply a keeper before the magnetising core is removed, and to keep the keeper in place until after the armature has been replaced. That is what we do, and the result is as good as can be achieved using an external remagnetiser.

Of course, there are huge benefits of an internal remagnetiser, which is far less wasteful of copper, iron, money and bench space. Going by the wire gauge and resistance of Magnetoman's rig, and if my calculations are correct, it includes about 30 kg of copper. By comparison, an internal remagnetiser core takes about 300 g. Magnetoman's rig weighs 'several hundred pounds', whereas an internal remagnetising core weighs less than 2 pounds. And yet both can drive a magneto's magnet fully into saturation.

Brightspark Magnetos