Rich as you already know with Slingerland and Ludwig almost anything is possible.The orignal 1Ns in the '60s were 22, 13, 16...then in the mid '60s they cataloged them with 20" bass drums until the '76 catalog. If the tom on the poster's set has a tom mount, the set may have come as part of the Duet outfit, which for many years had two virgin 20s.
If the 13" was sent out of the factory with no mount, that would be pretty rare.
Beautiful looking set. Slingerland was my 1st two drumsets. Blue satin flame,-mid 60s last set. Drafted Army. Came home after time served, rejoined band and played the Slingerlands for many years more. They had a sound that worked with every music style. Enjoy Angelo. I know you do!