Extra piece on the Yugo M64

Does anyone know what the little piece that goes into the left side of the Yugo M64 mag well is for?

It's for using with the special M64 magazines to hold the bolt open after firing the last round from the magazine.

It's for using with the special M64 magazines to hold the bolt open after firing the last round from the magazine.

I did manage to find some pics of the special M64 mag.
I have plenty of the Yugo BHO mags and they don't require anything special to work.
I even assembled a Yugo M64 with that little extra piece installed and regular mags fit great.
Was there something that was supposed to clip on to the rifle to work with the special mags?

The M64 magazines were before the M70 magazines. With the M64 magazines that piece holds the bolt open.

Yugoslavia was just copying what NATO mandated. Thinking it was an "improvement" or upgrade. At the end of 1961 NATO mandated integrated 22mm (American not European) rifle grenade launcher muzzle device, last shot bolt hold open and brass case ammunition. Yugoslavia copied all of these things.