Good catch/correction 3B.

When I send my suggestions in the PM, I failed to carefully read in the title that Harold's acronyms came from a Master Parts Catalog.

Sorry, Harold

SBC = Sch. Bus. Ch.
http://chevy.oldcarmanualproject.com/parts/1929_54/006.HTM

Also, D-D (Dubl-Duti) was also used to refer to Forward Control Chassis.
http://chevy.oldcarmanualproject.com/parts/1929_54/005.HTM