Safety undercut + compression sleeves in paracorr barlow or focuser = a real nuisance.
I would say the undercut was as poor idea as the "instajust" feature.
Here is a story that could be worth reading:
My 13mm and 8m Ethos's are equipped with the TV extensions so they're 2 inchers. This is all fine and dandy but the eyepiece barrels themselves have the notch for the setscrew.
Exit the Paracorr type 1, Enter the Paracorr type 2.
The Paracorr 2 allows the eyepiece to inserted somewhat deeper, deep enough that the compression ring rides on the eyepiece barrel and not the extension. In fact, it is exactly the right depth so it lines up perfectly with that cutout notch.
Well, that is what happened the very first night I was using my new Paracorr 2.
And when that happens, tightening down the set screw deforms the compression ring into the notch.. and brass, being relatively ductile, it does not spring back..
That eyepiece was stuck, the brass had the eyepiece trapped. At first I didn't understand what had happened but when I got the whole shebang under a bright light, I realized what the problem was and I realized the solution, unscrew the eyepiece from the extension tube, this releases the eyepiece.
Then remove the deformed compression ring and toss it in the trash. This releases the extension tube.
Final step, replace the setscrews with either brass or nylon. I had some 10x32 brass thumb screws in my spare parts so I used those for about 2 years. I recently found a surplus store that had thousands of 10x32 nylon thumb screws and I was able to buy about 18 of them for a dollar so now I've got Nylon Thumbscrews in my Paracorr 2.
If the eyepieces hang up on the thumbscrews, I know what has happened and I just back the screws out.