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Here we have the Zastava M70AB2, and it exhibits some typical features which Yugoslavia was very fond of in their Kalashnikov clones.
Yugoslavian doctrine was big on grenade launchers, specifically the kind which is part of the muzzle of a rifle; a grenade is fitted over the muzzle and a high powered blank cartridge is used to launch it.
Their AKs would generally be built very particularly for this, a lever fits over the gas-block, flipping this up shuts of gas from the piston, it is usable as a sight for ranging and aiming the grenade as well.
A single blank cartridge is loaded in the chamber (possibly a special small magazine of just blanks is used for launching many grenades in succession), and when fired it will propel the grenade at what you're aiming it at.
Since grenades have FAR higher mass than the typical 123gr .311 caliber bullet, this means massively more recoil coming back, and it also means more force on the rifle.
The gas being shut off by the lever makes sure all gas is used to launch the grenade, but it's also to make sure that vastly increased pressure coming back doesn't cycle the action, which at greatly accelerated speeds can cause *incredible* wear and damage the gun in short order (the American M1 Garand rifle has a similar concept with its grenade launcher attachment shutting off the gas).
Still, as launching grenades is an important part of Yugoslavian doctrine, the rifles are typically built with a stronger receiver to handle this, using a slightly thicker gauge of sheet steel, and using a thicker trunnion (bulging visibly beneath the ejection port), the added strength is to ensure you can shoot lots of grenades without worry.
The enlarged trunnion is emulated from the Russian RPK, the squad automatic weapon variant of the AK, where the receiver and trunnion is reinforced in a similar manner (among other changes), to ensure it can be used for high volumes of suppressive fire.
