"GB" is best translated in English as "gas braked". The barrel is permanently attached to the frame, and the slide ain't locked to nothin'. Yet it fires all sorts of 9mm ammo, and is quite pleasant to shoot. Add in a 18-round magazine, and you almost had the standard Austrian service pistol, except Gaston Glock came by and spoiled everything.
Here's how it looks, LNIB, complete with all the paperwork, and the 10 meter test target, dated August 1982. Production stopped in 1984, about the time the Glock appeared. As you can see from the "good ol' days", packaging did not have to be as up-armored as is the case for today's handguns sold on the American market. Just a cardboard box, and a green flocked plastic tray with some nice cleaning gear. I've also seen pictures of the tray in red.
With the slide retracted, the recoil spring guide is a tip-off. It's both a spring guide, and an exhaust pipe.
When field stripped, the barrel, hard-chromed inside and out, is the big tipoff. I put a drill bit through the hole that goes right through the barrel of what is, for all intents and purposes, a fixed piston with a moving cylinder surrounding it.
The "piston" seals up the front barrel bushing/gas plug, creating something like a gas-filled shock absorber. A tiny hole in the spring guide/exhaust pipe bleeds the gas forward out of the weapon. This is a gas-retarded blowback design. The more pressure the round develops, the more cushioning in the gas "cylinder". There's hot gas inside and outside the barrel, hence the hard chrome all around.
The gun is pleasant to shoot, and Steyr advertised that it could even safely handle high-pressure SMG ammo. The fixed barrel and hammer-forged polygon rifling makes it pretty accurate, as the test target shows.
It's not apparent at first, but the frame is stamped sheet metal. The seams are ground down, the frame was parkerized, and then given a crinkle finish. The trigger guard bow is plastic. I've had it for 25 years, and didn't notice that until tonight. Of course, I put one box of ammo through it, cleaned it, and (unintentionally) let it increase in value as an oddball collector's item. Asking price, for the few seen on Gun Broker, is in the $1000 range.
The overall design in that of a standard DA/SA autoloader with a hammer drop, rather than a safety. It's an ingenious but practical design, and could have had a lot going for it, except Gaston Glock came by and ate a lot of people's lunches.