I've long wondered about this. My DM "number" is in the form NNLNLNN, where N = numeral and L = letter. The first three characters are 50X (the second character is a zero).

Problem is I'm not sure when I joined. This is because I had a US account back in then late 80s/early 90s, but for several years didn't fly US. Then in the late 1990s I started flying US again, but my "new" DM account had several thousand miles in it, presumably from flights I took eight or nine years earlier.

Whenever I enter my DM "number" over the phone I have to use the numeric isomorphisms for the letters. This implies that some all-numeric sequences are blocked off.