hah this is where my hobby of hoarding boarding passes comes in handy for once.

pmua didn't have them.

pmco had a number in the upper right of the ticket which always seems to change, i'm assuming that's the sequence number. if you look at tickets today on the ticket stub you'll find it under your name, to the right of the date. on the main part of the ticket it's on the right end of the row with the date. it's in the same place where current UA puts them on the tickets.

previous generation pmco tickets (the gold stripes in the middle) had them in the same place.

us air has seq written right on the ticket, hard to miss.

edit: a little off topic but looking back on it and thinking about people today complaining about the current ticket layout, the old one (the pmco BP with the gold stripes in the middle) weren't all that hard to read. i wonder why they switched from that. most of the information you needed was relatively uniformly sized and the important information (your gate and seat) were in the middle of the gold stripes. not the perfect BP but it wasn't terrible. hah and i just flipped past a pmco BP with the current day mixed wacky randomly sized font printed on the older gold stripe BP stock and it looks... so bizarre.