I know on a 1600/180b trailing arms which is nearly identical to a 240z/260z there is a letter A, B or C stamped on the top on the bearing housing. This corresponds to a A, B or C on the spacer. Different letters are not interchangeable. Some spacers are larger to adjust the crush on the bearings.
It may be the same on the Z rear hubs. I think it talks about it in the manual