Are these the drums? https://reverb.com/item/17353618-slingerland-maple-with-inner-oiled-sealed-sheels-1975-black
The sentence structure is one long sentence! So, I'm not surprised that they said "sheels"!
If so, it looks like the bass drum is a 5-ply. Not sure about the toms, though, which could be 3-ply or 5-ply. Normally, you would order the kit in either 3-ply or 5-ply. If you're interested in this kit, try to find out what the shell makeup is for all the drums. Also, the 5-ply always seemed to have more lacquer on them. May just be who was doing the spraying at the time.
Another word of caution; I've never been a big fan of these thicker 5-ply shells. The bearing edges are super sharp and demand that the edges be level to avoid buzzing. Slingerland just couldn't seem to get level edges during this time period, despite the fact from catalog pictures showing a technician leveling the shell on a flat table using a light! One way to cut down or eliminate this buzzing was to use Pinstripe heads! I had a BR 80N kit with 5-ply shells and it was the worst set of Slingerland's I ever had! Took too much tuning to get them to sound good. But, I was able to get them to sound good!
-Mark