Ok so bear with me a bit here, tearing down these carbs to clean them all up and found this, which I found out to be the accelerator pump, the square part with rubber, but this triangle shaped spot has threaded holes, but there was nothing there... am I missing something?
The triangular vacuum sensor wasn't used on the 81 KZ650-CSR, so it wasn't machined at the factory to accept the sensor diaphragm & didn't come with the triangular cover. That puzzled me also as the K&L kit includes the sensor diaphragm.
The carburetors on the 81 650-CSR are VM24s. The engine side measures around 26 mm, but inside the bore near the throttle slides is 24mm.
Other accelerator pump stuff. Each carb bowl should have a ball, weight (rod) & clip and each of the pump nozzles on the carb bodies should have an o-ring. I used o-rings from a HF metric nitrile o-ring kit.
Edit: you should test the pump to make sure the fuel squirts straight thru the carb bores without hitting the walls.