nah, the only real difference is that the Mk II is surface mount and the mk IIa is thru hole. OK, so the mk IIa also has a keyed shaft and is from a different manufacturer, but electrically they are the same. If you are really clever you could probably fit a mk IIa switch to a mk II as the pin pitches are the same ...
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(list 6284, Mk1 S/N 00299 4GB blue [sold]. Mk2 S/N 080000094 20GB blue)