As the title says, this is an SB Tactical SBTi brace for the B&T APC.
A3 tactical also makes adapters to use this brace on the Stribog, MP5 family, and Scorpion Evo: https://a3-tactical-inc.myshopify.com/collections/stocks-and-stock-adapters
It’s new in the package, never installed, black in color. Asking $50 shipped, payment via Paypal.
Pics available upon request, and may post some later today regardless.