Please help me identify this splindle tooling system

a)

This spindle is a Colombo RV90.2 SB. Spindle taper look like MT2 but it is a blind hole. Nut have a threaded hole at the bottom.

Attachment 450162

Attachment 450164

b)

What it the use of that threaded washer that I have unscrew ? Does it mean that the MT2 socket can be removed ? I can't find an exploded part list of that spindle. Someone can help ?

Attachment 450166