Eric and Bill:
Interesting mystery, and it leads to more questions. It doesn't appear that those missing numbers were ever filled. It looks like the Walthers people made an error <SHOCK!> . . . something I'm sure that almost never happens.
In my 1958 ORER the KCS 20800 series is broken into 20800-20801 and 20802-20809. The former pair are now XAP boxcars covered under note "S" which says they are equipped with special racks for loading Chevrolet front axles, designated as Circle 3 [?] loading. The "3" is partly blatzed out in my photocopy, so I'm not 100% sure of the wording. 20802-20809 remain as XM boxcars. There are no other cars with a 50' 6" IL, 10' 4" IH and 14' 6" door openings on the roster. However, there are 189 cars of virtually the same dimensions listed on sister line Louisiana & Arkansas as 36001-36200 except that they have a 9' door opening. Could these two groups be related?
Now series 20851-20900, 50' 6" Il, 10' 6" IH with 15' x 9' 10" door openings, should contain 50 cars but has only 14, plus 20859, 20866 and 20880 listed separately as XAP and covered under note S as above. That's just 17 cars. No other KCS cars match these dimensions. Hmmmm. Where are the other 33 cars? Also interesting, though not uncommon, series 20851 were originally XA and equipped with auto racks and chain tubes.
Yours Aye,
Garth Groff