That's a yugoslav rifle grenade.
Here's more.
This is an inert yugo rifle grenade next to my yugo SKS.
Yugo rifle grenades come with to blanks in the tail boom, secured by a plug. One blank is 7.62x39, one is 8mm mauser. They are grenade launching blanks, not to be confused with normal 'training' blanks used to simulate firing with a blank adapter.
There are also 'bullet trap' grenades, that allow the firer to use his normal ammo to launch the grenades. These are a bit safer for the user (you don't want to fire a bullet into a non-bullet trap grenade), but the bullet trap adds weight and reduces range.
As I recall, the polish grenades are shot off of a 19mm spigot, where the Yugo grenades are shot off of a 22mm spigot. 22mm is the nato standard, so you can launch Yugo grenades off of any NATO spec flash hider/launcher (like the A1/A2 flash hider on your AR15). There are many others, such as the STOLL devices on FALs, the Belgian combo device, and others. I think the G3 hider is 22mm as well.