Bruce Smith offers this clarification:
The Wegman/RC kitbash models an R-30-12-16. The R-30-12-16s
were rebuilt from R-30-12 cars, to R-30-12-9 specs so the RC R-30-12-9
(or R-30-9) is the appropriate raw material as it yelids the correct
height body. R-30-13-12s were also used for R-30-16 conversions so for
the more adventurous, the R-30-13-16 variant can be modeled with the RC
R-30-9 body, the Wegman roof and either a Wegman (R-30-18) or Tichy
underbody (R-40-2/4) . If you have a Wegman R-30-18 kit, you can use
the leftover RC underbody to model the R-30-12-18, instead of the
R-30-13-18, which that kit models.
Maybe I'll just stick with steel PFE reefers.....