It's the Gene Krupa Sound King snare drum! They were made by the 1000's and were usually the standard snare drum with the majority of cataloged kits. The "snare extension" you are referring to was always on the drum. It's a hold-over from Leedy snare drums. Slingerland liked it and used it on the GK snare drum and also the Artist wood-shell snare drum. The snare wires used, up to around 1968, were an odd arrangement, where the butt end extended over the bearing edge, and many times a real hassle! Rather than explain, I'll post some pictures. By sometime in 1968, the Zoomatic was changed to include a wide clamping bar so Slingerland could install normal length wires with filament tape strips at each end. A much better setup!
As to the GC snare drum, I feel that their asking price is too high. You need to be familiar with the Zoomatic and know what to look for to be sure it's in good physical condition. Zoomatics in good working condition can be hard to come by!
-Mark