Frank,
Thanks the photo really helps.
As you have seen on your Optar, the section of the shuuter barrel that has an internal thread to take the rear lens section, is typically threaded externally to thread a retaining ring onto.
Yours looks smooth with no external thread, which is that I have seen with oscilloscope lens shutters. In those applications, the shutter was held in place with a clamp and certainly there are a number of suggestions here on how to fix that.
I do not know why they clamped them in on oscilloscope cameras rather than thread them in (would manufacturing costs be that different?), unless maybe the clamping also allowecd them to do a fine focus prior to tightening the lens in place...
Now you need to take a look at the aperature scale...
Good luck,
Len
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