Never heard of that, either.
For most of their history, Sunn used a labeling convention that went as follows: number of drivers in the cabinet, size of the drivers in inches, suggested use. So a 610S had six ten inch drivers, and was suggested as a speaker, not dedicated as bass, guitar or PA. If a special driver was used, or the cabinet had a special configuration, that would take the place of the 'use' letter. A 118H was a folded horn cabinet, a 215M was a 215S loaded with Magna drivers. PA columns were designated SR, for sound reinforcement. A 412S was a 4x12 speaker cabinet, but a 412SR was a PA column.
They started going away from that convention with the Magna PA cabinets in the late 70's, though.