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Which firmware version are you running? Check in DuetWebControl (Settings, General or Machine Specific) or with M122. This was changed in...
The tool offsets are applied between tpreX.g and tpostX.g. See https://forum.duet3d.com/post/155639
Externally 17x15 mm, internally ~13x10 mm, 15 chain elements + 2 end elements
The gears are not available separately. So you need to buy the Hemera bowden kit, the heatbreak and you will have only the bowden adapter as...
The E3D Hermes Extruder https://www.jayuk.org/e3d-hermes-extruder/ seems to be designed with the tool changer in mind. It looks a little wide though .
The manufacturer was IKO. The model number is not specified but IKO has probably linear rails with similar or the same dimensions as Hiwin mgn/w....