Jim,
I can't answer your question directly because I don't know what a passenger car jack looks like. However, I will attempt to give a short verbal description of a '56 Bird jack.
Base: Has a square shaped 'hat section' that accepts the staff. It is marked with "AB-L" and "A" on the opposite side. Markings are impression stamped.
Staff: All Bird staffs are 31" long. The '56 is a modified 'T' cross section, with the cross bar of the 'T' being dipped in the middle.
Mechanism: Pretty straight forward with no particular characteristics that come to mind.
Hope some of this makes sense to you. Tom