In reality the Skylab B hardware exists and is on museum display. It was identical to the flown Skylab, since it was its flight spare.
For this add-on, an alternate reality is considered, based in several proposals by NASA made in the 1970's.
The main changes are the lack of the Solar Observatory, the additional docking ports for Soviet spacecraft, and the alternate solar panel positions. This configuration would allow the station to accommodate extended docking or servicing by Apollo, Soyuz, Progress, Salyut and eventually the Space Shuttle.