I signed up two family members for CO OnePass in late 2009 about a week before CO entered Star and the UA-CO codeshare ramped up. They both received 2L6N accounts.

The rest of us never had CO accounts, so we got the 3L5N format when the airlines merged in 2012.

It’s interesting to see how similar some of the numbers are based on when they had accounts created. They’re all bunched up with a set of the same letters and first few numbers. Two family members signed up for UA MP in the early 80s, two who signed up in the late 80s, and some who signed up in 2022.