OK, for the benefit of anyone else who might in the future tackle an Agifold, I can now answer my own question:
The range-finder (and presumably the viewfinder on the basic model) on the cameras where the assembly doubles as the door catch,is held in place with a single screw, accessed from above, under the covering.
To remove the covering required alcohol (I used methylated spirit). Lighter fluid didn't touch it at all - does this indicate it is shellac?
A very sticky, gooey mess it made, too. After that it was fairly straightforward to service the range-finder.
PS. Still haven't noticed the shutter button to be stiff on any of my cameras. I reckon either the shutter or a bent linkage must be to blame...