Sorry I dont know the names to all the parts, but everything works on mine but when i press the button down to change out the heads it doesnt work and i cannot tighten or un-tighten it (though currently un-tightened)
Tools & Hardware - Others - Dremel - Variable - Speed Multipro Rotary Tool 395
Answers & Comments
Contact them direct at dremel.com and you can download a schematic which I think the locking collar/mechanism is rounded but they also have a toll free number for technical support.