Yeah, was thinking the same thing. At some point he switched to rounds but I think the first two albums at least were with flats.
He didn't switch to rounds on his Jazz until after Zeppelin, per an interview he gave Guitar Player Mag in 1977. His single coil P, his Alembic, and his Bec Var 8 string were all rounds. That said, the first two albums, as well as The Song Remains the Same, are the only albums that I think can be clearly attributed to the Jazz bass and no others. After that, he started mixing things up a bit in the studio.