Bruce Langhorne's contributions are credited on the record labels and sleeves, and for a few of those earlier years he was the only other guitar player in the studio besides Bob, so his playing is very easy to discern. (In most cases, he's playing electric with Bob on acoustic, so there's little doubt about who's playing what.)

With the release of Highway 61 and then Blonde on Blonde, there were multiple guitar players listed in the credits and a much more complex instrumental mix, so then it became harder (well, impossible really) to recognize the players.

Nowadays, I can recognize Robbie Robertson's instrumental "voice" easily enough, and can hear him clearly at various times during B on B. But back when the album first came out, I had no idea who any of those guys were, and often wondered about the guitar players and the guitar playing.