Hi Jeff!
To answer your questions, here's what I can say. No, Slingerland never installed dual mufflers as standard. Are you talking one muffler on each head? If so, the lower muffler was probably installed by the owner or other drum shop. As to it being a Duet, the fact that there is a missing bass drum no longer makes it a Duet, does it?!!
As an example, attached is a picture of a 1967 Slingerland kit I purchased some years back and still have. It was originally a Duet, but the seller sold the other bass drum well before offering the rest of the drums for sale. The 8x12 rack tom you see with the kit was offered separately as a shell only, with no hardware. It was the left-hand tom. I purchased the bass, floor tom and right-hand tom together and had to get the matching snare drum and left-hand tom shell separately. Moved over all the hardware to the left-hand tom. Since there was no second bass drum, it really is no longer a Duet, so I chose to install a Maxwell rail mount. Not only that, but finding an original Slingerland double tom stand, with ratchet mount, would be next to impossible! So, I use the kit as you see it and never worried that it was a Duet. Hope all this info helps!
-Mark