Originally Posted by
WineCountryUA
IIRC, 0061 is/was correct. CO-origin planes had fleet numbers starting with a leading zero. The fleet tracking system they went with (apparently from the UA side?) didn’t like leading zeroes, so CO planes had their fleet numbers changed to start with a 3. Most, if not all, fleets from pmCO (including, IIRC, 737NG deliveries post-merger) will show up as 3xxx in the app, but have 0xxx on the fuselage.