Ted,
The cars up at Athol were never in Iowa.
They were bought by Bill Harrah (Harrah's Auto Collection) and moved to Sparks, Nev. when the SP scrapped the Owens Valley narrow gauge. Then after Harrah's death much of the auto collection and other Western Americana were auctioned off. At the auction the owner of Silverwood theam park at Athol bought the three box cars, the ex-Eureka-Nevada 2-6-2 and the ex-SP combine #8. Other ng cars that Harrah had ended up going to the Nevada State RR Museum. In time the combine #8 was given to NSRM.
The box car identified by Randy Babcock as MW 354 was originally box car #354. It was known for years as tool car MW 1C (part of the relief train for the manual derrick MW 1). The car continued as MW 1C until the end of the narrow gauge.
Looking at Bob Bader's book on the SPng equipment the other box cars should be SP 315 (45 after 1946) and SP 450 (12 after 1946).
Brian Norden