Yes, you’ll need the K19 Back Flap Cummerbund Adapter to attach the Warfighter Cummerbund to your K19 Plate Carrier. This adapter allows you to use the Warfighter Cummerbund or any other hook-and-loop style cummerbunds with the K19 Plate Carrier.
The K19 Back Flap Cummerbund adapter: https://agilitegear.com/products/k-19-plate-carrier-back-flap-adapter?_pos=1&_psq=ada&_ss=e&_v=1.0