When it came time to number these cars I had to do a bit of research. I found out that just about all the freight cars on the SP were acquired from other railroads such as the Carson and Colorado. Also, in 1946, the SP re-numbered all it's narrow gauge rolling stock. Prior to 1946, the boxcars wore a three digit number. After 1946, they were all changed to two digit numbers. The other big difference was the herald. After the 1946 re-numbering, the herald was not reapplied.
The SP narrow gauge historical society has some excellent resources online. Researching their archives was especially helpful: www.spnghs.org
Type I Cars
Type III Cars