quick rule of thumb for you!
The bottom racks for the 15,16,17 are all basically the same and will interchange fine. The 15 original rack lacked the drop down tines at the back end of the middle section that the 16 and 17 came with.
The top racks for the 16 and 17 interchange but the 15 had a unique track and roller system with notches to adjust the rack by setting the rack in one of 3 different notches in the slides to adjust the height.
The 18,19,20,21,22 series lower racks are nearly identical and interchange. The only differences you may find are the number of rows of tines on the earlier series were greater and also, the placement of the side tines varied from model to model but in the end, I would be nearly 100% sure that WP made only one style available to fit all models. Just like the 18 upper rack was quickly superceded by the 19 style rack with the longer wash arm. And the China Guard starting with the longer arm style was wider in diameter.
The 23 had the upper rack with the drop in the front center rail which made pulling out the rack easier as well as a flatter front section of the top rack otherwise it looked like all of its predecessors. But again,fully interchangeable.
Now just for kicks, I sold an 18 and up upper rack to a 17 Superba owner and the dimensions of the rack were nearly identical. All he had to do was remove the spray arm on the underside of the rack and the China Guard and it worked like a charm in his 17 series machine. The adjusting mechanisms on the sides were identical to the originals in operation.
I hope this helps to help and not confuse!
