DEL, sometimes labeled RUBOUT has a very important feature. It's all ones. When punching a paper tape, if you make a mistake you can mechanically backspace the tape (there's a button on the punch rather than the actual backspace key), you can then press RUBOUT which overpunches the incorrect character. Presumably, whatever system that this was designed for disregards those characters when encountered. Amusingly, we though the HERE IS key was just to generate leaders because none of our teletypes had the drum programmed. Later I found out that you could break the tabs on the drum and have HERE IS send a short string of characters. ^E (called ENQ or sometimes WRU ...for who are you) triggers this to be sent in response. To get back to the UNIX tie in, I actually had for years a Model 37 teletype. This was one of the few terminals that you didn't have to set the nl mode mapping for. It had a large key marked NEWLINE where RETURN usually is and sent ^J (\n) and responded to it the way UNIX expected. In addition it handles all the ESC-8 ESC-9 etc... codes that nroff sent by default without needing a filter. Mine was an ASR so it had the tape unit. It lacked the "greek box" that the one at JHU had to print greek characters after an ^N (shift in). The thing was amusing as it didn't turn on the motor until the modem came ready and when carrier detect was asserted a big green PROCEED light lit on the front. It was quaint, but when I finally got a higher speed modem, I switched back to using a CRT. The Model 37 was a screaming 150 baud. I finally "donated" it to RS who dumped the thing behind someone's car somewhere.