I've just asked for an opinion and it isn't terribly informative. They can think of no other model that has this set up.
It's agreed they differ to the 39/54 in this regard. We've never done one, and although it would be apart quickly if it were on the desk here, its hard to tell by photos. The suggestion is that clip may be attached to an internal assembly like the cable rewind, and may only release if the screws on the side are undone.
The other [less helpful] suggestion is it may clip in, but until you do the first one you never know where the clip is till you have broke it to find out. Which isn't ideal if you only have one.
BTW, by motor number I meant from the motor inside the housing. Unless its a sealed case digital motor in those? In which case you swap out the full motor housing as you appear to be doing.