The difference of the z-axis profile length I explained in another post is only 2mm but led me to another conclusion about the belt drive. While the belt drive system on the MK3 has a self tensioning function realized in the grey plastic cap with a spring the MK2 version doesn´t have this. Due to this fact the belt on the MK2 has to be mounted already with tension to prevent backlash in the movement. Exactly this leads to the length difference of the aluminium profile. If anybody knows some preferences of the spring I would be happy to know and reorder.