HP QEDS encoder

I found a couple of motors at the local markets with HP QEDS5977 encoders on
them.
Can anyone offer advice on how to remove them from the motor?
Part of the case (axially aligned with the shaft) can be rotated to one of
two positions by a slot, can't work out what that does.
Carefully prising the case open looks like it'll take the optical unit with
the outer (back part of the) case, I'm worried that this will damage the
encoder wheel... Any advice would be gratefully received..

PK