The tube is held in place on the whip by 2 set screws and its adjustable. Here are some pictures. I don't think you could add one because it sits in between the coil that is not removable and the bottom threaded thing, which is not removable.
There was a 700 series that was basically the same as the MON-58 but I can't find my paperwork. I have several of these antennas and one was a 700 series.
You are correct. I remember the instruction sheet that came with mine (MON-58) or was it (MONR-58) and it had the same warning about not altering the position of the cylinder (Choke) as is shown in the image scan that @trp2525
I remember some of AS's MON series had a letter R suffix after the MON letters.
You may have sold me this antenna as NOS prcguy! I've had it quite a while and it still performs very well.
After seeing your picture, I think I do remember the set screws. And yes, adding the cylinder choke would require knocking the threaded part off the bottom of the whip and then figuring out how to reattach it. I suppose one could drill and tap it for small set screws.
I think they pretty much used simple tight slip compression fits for their MON series at least. Probably assembled with a press except for the little ball at the top for the low band adjustment on those models. You could remove and reattach the top ball after cutting the whip for the proper length.