Back a bit I collected three Isolettes. A 1937 dual format model (rebuilt now), and two Isolette IIs. One had the Apotar lens that was frozen by the "green grease" - now fixed using spring clamps (of all things). The other had no lens or shutter. I think it was a brand new body that had never had a lens or shutter mounted. Cost $15. A few weeks later A Solinar shutter/lens combo showed up for $20. Cleaned and lubricated that and I had a working Isolette II - with one problem. I could not see through the red window at all. I tried flashlights, high intensity lamps, sunshine and multiple types of film. My wife (better eyes than I) thought she saw a "5" once. I thought "this is stupid - I want to use my $35 camera". I know this a bit drastic, and there are threads about this problem going back decades - effecting resale value etc (who cares?) So I tried to push the window out, and found the force required might damage the body - didn't want that. Xacto did the trick. btw the other Isolette II window is a much more translucent orange - instead of this camera's almost opaque red/black. I think the original manufacturer (Agfa) used a press to install the window/door combo. Pretty sure that pulling that apart would damage the finish, and possibly the metal parts.
The offending bit of plastic
Hole had to be enlarged, but the remains slide around within the window - jagged
Some trim remains of color filter printing gel - .50 Magenta
Marking the window size with a sharpie
Initial trim (needed more) so I could slide it into the metal frame on top of the old plastic remains
Now tucked under the metal frame - sliding the metal door shut should help it into final position
Just noticed the Solinar scale is in feet, and the Apotar is meters. Sackem-razim - that means my couple test shots on the Solinar are going to be out of focus, and are useless to compare to the Apotar. Must remember for the next ten frames