Re: NKP Boxcars 20000-20500
The short answer to your question is yes. The box cars in the subject line "NKP 20000-20500" were of mixed designs. NKP 20000-20499 was comprised of two orders of different box cars, neither of which were ex-WLE. Whereas NKP 20500 was the first car number of a series of former WLE box cars originally numbered 26000-26199 built to the same specs as NKP 20000-20199 (see below), which from ca. 1952-1960 were reassigned as NKP 20500-20699 and leased back to WLE.
NKP 20000-20199 (200 cars) were built new 1-40 by Pullman-Standard in lot 5604A. These cars were recently discussed on the STMFC and were built to a P-S lightweight proprietary design of primarily welded construction to include 18 side sheets per side.
Conversely, NKP 20200-20499 (300 cars) was built 2-41 by ACF in lot 2080. They were an ACF lightweight proprietary design and primarily welded construction to include 14 side sheets per side.
Patrick C. Wider authored & published an extensive 265-page, two-volume edition of Railway Prototype Cyclopedia Vol. 31-32 to include both series of these NKP lightweight box cars. These NKP cars were examples of thousands of steel box cars of various lightweight designs built during the 1930s, 1940s, and well into the 1950s as described & illustrated in the book.
To your other point NKP & WLE also purchased numerous 1937 AAR box cars, which are covered in Pat’s comprehensive 385-page book on box cars of the 1937 AAR design published in RP CYC Volume 35. Also, a roster of 1937 AAR box cars is available for downloading on the STMFC web site at http://www.steamerafreightcars.com/prototype/frtcars/protofrtcarsmain.html