There's a bunch of auctions on eBay for some 1920's glass plate photos of commercial and railroad scenes. This one:
says it's a South Pacific Coast boxcar being loaded at the "Oakland Shed"
The wheels and grab irons look scaled correctly for a narrow gauge car, but the "SPC 81319" reporting marks seem odd - I would have assumed the SPC stayed with its system of low numbered freight cars.
Has anyone else ever seen 5 digit car numbers on an SPC car?