"Cornerless Italian & Spanish Basses"

[Scott Pope]

Speculation: if the ribs were narrowed, it would seem to me more likely that a luthier would narrow them from the back; otherwise it would take a lot of re-engineering of the neck block instead of just a trim of the button side of the heel, which unless there were grave structural problems with the "top" side of the ribs would not be a cost effective modification to the bass, or any bass. And if it were trimmed from the rear, then the peg, assuming centered originally, would be farther back than forward.

I vote for it "always having been that way."

Please correct me if my reasoning is not sound (pun intended).