"I wonder what other brands besides FWB and Walther made SSP's?"
Some that come readily to mind include Air Match and Pardini, both pistols. Also, the Titan Mohawk rifle came in single or multi-stroke; I've owned both.
In fact I set an NRA silhouette National Record in Sporter Class with the Titan SSP back in the 1980s. Whenever a piston-gun shooter wondered about it, I'd let them shoot it; the first shot after charging the rifle for them (I have the strength of ten men). Invariably they'd say something like, "WOW, that's almost like cheating!"
Then I would instruct them carefully in how to charge, cock, and load it; whereupon only some could actually do so. Then they'd say something like, "DANG, how can you shoot a forty-shot match with that thing?"
My reply never varied. "It's not just shooting a forty-shot match with it that's the problem. It's shooting a forty-shot match with it AFTER shooting as many shots in practice that's a bitch!" That always ended all talk about cheating.