Another view of why "Toten" is not an ideal abbreviation:
Totenkopf is a composition of the words Toter (dead) und Kopf (head), or to be more exact: It is the composition of the stem of Toter (Tot) and the epenthesis en and Kopf. The result is Totenkopf. Note that en is an infix, an addition of a sound and not a word on its own or a part of Tot. Its function is to connect two words, but Toten doesn't have a second word. So to me, it looks odd and doesn't even look like an abbreviation, it's just truncated, ending with the en infix.
Toten (or with a noun marker "die Toten") is also the plural of "der Tote" and means dead people. This is probably what Hanny is thinking about. I don't, the grammatical case doesn't fit in that case. Maybe this makes it odd too.
Using "Tot" is bad too because it looks like a misspelling of Tod (death) and not like an abbreviation of Totenkopf. TKopf is the best I can think of. Or SS TK or SS T would be fine for me, but I lack the historical background and don't know if it is historically accurate.
