Well done, iffig.. that was quick. There only ever was one prototype of the Schelde S-20. It was designed as a trainer and liaison aircraft, but it's main purpose was to gain design experience with the twin boom pusher configuration and as a proof-of-concept for a follow-up fighter design for the Dutch military, the Schelde S-21. Drawings of that follow-up design can also be found on the website you referred to. I guess the a, b and c designations on that website are just to distinguish between the Photographs..
Unbekwown to the Schelde designers, Fokker was working on a similar fighter project, the D-XXIII, which used a two-engined push-pull configuration. Although the designs look similar, the reason for the unusual design layouts were different: De Schelde just wanted an unobstructed forward view, so it could fit some quite heavy armament in the nose of the aircraft without bothering with the arc of the propellor, just like Fokker had done earlier with the G-I. Fokker tried to mount two engines on the centerline of the aircraft to reduce drag and torque.
Both designs had a serious flaw though, in that it was nearly impossible to bail out of the aircraft without being hit by the rear propellor. Fokker was experimenting with an ejector seat to deal with this issue, just like Saab did in 1943 with the Saab-21. The Dornier 335 Pfeil later had a similar issue which was dealt with by using explosive charges to eject the rear propellor, but that was not an option with a twin boom design as the still rotating propellor would have crashed into the tail, sending debris off in all directions and quickly deteriorating the flying characteristics of the allready out-of-control aircraft.
The Schelde S-21 would have had another setback as a fighter in that it offered no forward protection to the pilot at all. It's not a nice thought chasing a bomber knowing that all that's between you and the tail gunner is the glas of the cockpit.
In the summer of 1940, the S-21 design caused a bit of a scare in England as it was mistakenly attributed to Focke-Wulf and expected to soon be in all-out production: http://up-ship.com/blog/?p=8134
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