Oh it's very much a bass. I've read that he brought back the Hofner into the recording studio during '69, which is proven true by the studio and on-top-of-a-London-flat recordings(as seen in the move, Let it Be). That bass has a distinct sound. If you listen to the Beatles' Anthology, no.3, "I Dig a Pony", you'll hear that his Hofner has a dry, muddy sound that gets polished up(thanks to one Mr. Phil Spector--Lady Killer!

) , for the final release.
My other guess is that the treble is set rather higher than he usually plays it, and the gain is also high. Gives it that grungy sound.