Very strange. If you have the ability to post detailed photos of the shell and bearing edges, that would be enlightening. It would be a Radio King "first", at least in my experience. I thought I found one at the Chicago show a few years ago and I was convinced it was solid mahogany. Upon close inspection, it proved to be a three ply.
I'd love to accommodate, however, that'll be a little tough because I sold it to Nicko McBrain back in '88. I wonder if he still even has it. If anybody has access to him, it might be cool to find that out and get the most current evidence from him if he still does.
I was as curious as you because I was also unaware that Slingerland had ever offered a 1-ply mahogany snare drum. Needless to say, this one underwent quite a bit of scrutiny and research, at least what was available back in the late '80's, which was old catalogs and folks like Charlie Donnelly.
I was fortunate enough to have some catalogs from that era that I'd found rootin' around in the basement of an old music store. (Found a bunch of Slingerland calf heads with both Buddy and Krupa on their red and white boxes, along with a few Amrawco's, as well. And an 11" knotty-pine RK tom shell.)
Anyway, I also spoke with Charlie Donnelly who said he'd had a few come through his place through the years. So then, I was at least aware of the possibility that this could be one. All this was done before the drum actually came to me.
When it arrived, off came the heads. It had the sharper bearing edges that were typical of that time period. The heads tuned on the outside edge, of course, which was mahogany. A magnifying glass revealed the only seams in the edges being the ones between the shell and the glue rings. No poplar center ply (which I figured would be there) or any inner seams of any kind except for at the joint where the two ends of the mahogany board met.
Funny thing was, I never could quite get it to sound like my maple ones. Bernie Stone and I figured out it was the snare beds and I was about to send it off to Fort Wayne when the opportunity to sell it to Nicko showed up. I told his tech (big ol' rugby-playin' Brit named Steve Gadd(!), fer crissake. ONE HELLUVA fine gent while I'm at it. He snuck me back to play on Nicko's kit after soundcheck with the 200,000 watt P.A. still up. Unforgettable, but that's another story.) Anyway, I mentioned the beds possibly need touched up and he said they'd worry about it if need be.
In fact Nicko was talking to me about his possibly buying Bob Henrit's drum shop in London. I don't believe he ever bought it, however.
Anyway, that's the poop as far as I can take ya.