SN74AHCT1G126-Q1: Last Q meaning CAHCT1G126QDCKRG4Q

It's short for "Q1"; the name would have become too long otherwise. (For the same reason, "SN74" was replaced with "C".)

The devices with and without Q1 suffix are made at two different manufacturing sites, so there are small differences in the material content. There are no other differences; both are AEC-Q100 qualified.

(The "G4" has no meaning nowadays; see [FAQ] Why does a logic device's part number have an E4/G4 suffix?)