After Looking it up its a KAK Shockwave Blade... Its what you put on while you are waiting for your SBR stamp to go through LOL
https://www.wingtactical.com/firearm-parts/ar-15/buffer-buffer-tubes/kak-shockwave-blade-pistol-stabilizer/
Yup, pistol brace. IF they are waiting for the stamp to clear, then that's ok. I just have seen too many builds where they didn't want to stamp it for some reason. Just seems like cutting a corner at the end of the build.
Also for a while alot of people were shoulder the sig brace so there was no real reason to pay $200 tax stamp and wait a year for a form 1
But since atf deemed that as illegal it should start to change back.