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Funny, I couldn't recall "~" being important at all so had to go check. See https://codeberg.org/PDP-10/its/src/branch/master/doc/_teco_/tecord.1132 and while I do see a few uses of it, they seem very minor.
I read this into an Emacs editor buffer and did "M-x occur" looking for [~] and got these, all of which seem highly obscure. I think it is probably because in the early days there may have been a desire not to have case matter, so the upper and lower case versions of these special characters (see line 2672 below) may have once been equivalent or might have some reason to want to reserve space to be equivalent in some cases. Remember that, for example, on a VT52, he CTRL key did not add a control bit but masked out all the bits beyond the 5th, so that CTRL+@ and CTRL+Space were the same (null) character, for example. And sometimes tools masked out the 7th bit in order to uppercase something, which means that certain characters like these might have in some cases gotten blurred.
10 matches for "[~]" in buffer: tecord.1132
1270: use a F~ to compare the error message string against a
2017: case special character" (one of "`{|}~<rubout>").
2235: the expected ones, with F~.
2370: kept in increasing order, as F~ would say, or FO's binary
2672: also ("@[\]^_" = "`{|}~<rubout>").
4192:F~ compares strings, ignoring case difference. It is just
4446: this option include F^A, F^E, F=, FQ, F~, G and M.
4942: string storage space, but begins with a "~" (ASCII 176)
4977: character should be the rubout beginning a string or the "~"
4980: "~" or rubout, then it is not a pointer - just a plain number.
If I recall correctly, this also meant in some tools it was possible if you were using a control-prefix con CTRL-^ to have CTRL-^ CTRL-@ be different than CTRL-^ @ because one of them might set the control bit on @ and the other on null, so there was a lot of ailasing. It even happened for regular characters that CTRL-^ CTRL-A would get you a control bit set on #o1 while CTRL+^ A would get you the control bit set on 65. Some of these worked very differently on the Knight TV, which used SAIL characters, I think, and which thought a code like 1 was an uparrow, not a control-A. There were a lot of blurry areas, and it was hell on people who wanted to make a Dvorak mode because it was the VT52 (and probably VT100 and AAA) hardware that was doing this translation, so there was no place to software intercept all this and make it different, so that's probably why something as important as Teco treaded lightly on making some case distinctions.
But if someone remembers, better, please let me know. It's been 4+ decades since I used this stuff a lot and details slip away. It's just that these things linger, I think, because they were so important to realize were live rails not to tread upon. And because I did, for a while, live and breathe this stuff, since I wrote a few TECO libraries (like ZBABYL and the original TeX mode), so I guess practice drills it in, too.
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