They seem very similar. All I can think of is that Slingerland shells were possibly all maple(?) plies with maple reinforcement hoops...? I don't recall if Slingerland used poplar-core shells or if they used mahogany and/or mixed plies, ether. I also don't know what (if any) evolution there was in regards to Slingerland shells.
Whenever 3-ply shells are referred to, it's almost always a reference to Ludwig. But, weren't vintage Slingerland shells also three-ply?