The White Pass now has its roster on its web site:
[www.whitepassrailroad.com] look under company and then roster.
It does not show any 300 series flat cars.
I've seen one of the people involved on the White Pass discussion site post information on the NG discussion site. Maybe more information is available.
By the way, I have the Sumpter Valley numbers of the 10 flat cars sold to the Army back in 1942 for use on the WP&YR. Got these from Mal Ferrell way back in 1976.