Originally Posted by lincolnjkc

pre-merger UA MP#s were (AFAIK) entirely numeric and fairly long (~11 digits if the ancient MP magazine promo I for some reason still have laying around and my counting skills are to be trusted)
pre-merger CO OnePass#s were (AFAIK) universally 2L6N though I've never determined a pattern -- mine, opened in ~2006 starts with AP, while my wife's -- opened not that long after mine -- is UK, a coworker, who claims his account is late '90s is Z-something.

post-merger the 2L6N survived for a while (at least the immediate aftermath of the merger) before giving way to the 'new' 3L5N

I suspect in the old days CO recycled idle OnePass #s after a while of inactivity and they stopped that practice at some point (which may or may not have been congruent with the 'miles never expire' change) which also combined with a generally larger passenger base necessitates a larger pool of values.

As an interesting side note, if an agent digs hard enough they can find "all" of your FQTV numbers -- in my case a about a year ago they found my original UA# (which I created for 2 flights in college in ~2003 and then never used again), the 'new' MP number that automatically got created from that account as part of the merger migration (which is a 2L6N and I think T...something), and then my CO#. Since I merged the pmUA account into into my pmCO account the first two mentioned silently redirect to my pmCO account.