Reading through this has made me think, I'm pretty certain that my KTN, or at least the one I used on all my recent flights to the US, contains digits only and certainly does not start with TT. But I have gotten my KTN through GlobalEntry, so YMMV.

Checking on some booking I have in the future, I can confirm that the number I inputed does indeed only contain digits. (And yes, Pre-Check and Global Entry did work just fine on my past travels).

Let us know what SAS tells you.