The Saturn IB-B was a proposed modification to the Saturn IB rocket, it was to be essentially an uprated version of the Saturn IB using the new MS-IVB-2 upper stage powered by the HG-3 engine developed from the S-IVB and an uprated S-IB-A first stage. It was Proposed by Douglas Studies in 1965 but it never passed the drawing board.
External links astronautix.com