Looks like a Slide Fire bump stock to me.
“Slide Fire bump stock”
I think you’re right. Not just the space between the stock but that looks like the nob on the bottom of the grip that lets you switch from semi to full. You just pull it out and twist, either locking the stock or allowing it to slide back-and-forth.
I speculated as to that the other night. I’ve got one on an AK. Perfectly legal.