I was curious about some keys I hadn't seen before, so for anyone curious, here's the purpose of the "rub" and "here is" keys (from the ADM-3A operators manual[0] page 3-7 and 3-8):

RUB. When the RUB (rubout) key is typed while holding down the SHIFT key, a non-displayable rubout code (ASCII DEL) is transmitted to the computer. The cursor is not advanced and the character code stored in display memory is not overwritten. RUB is normally used to tell the computer that a previous character should be deleted.

HERE IS. If the Answer Back option is installed, typing this key transmits an identification message (stored in a special ADM-3A memory) that identifies the terminal and alerts the host computer that a message is to follow. If no Answer Back option is installed, the key has no function.

[0] http://www.classiccmp.org/dunfield/altair/d/adm3a.pdf