From your picture, the the one screw with just a slot on the end, is your high speed screw, and is missing the flat washer and O ring, like the other three. You do not need to order a new screw, I have never in my life, had to do that, unless the threads were messed( which has never happened) , Your kit will have the new O rings , but not the flat washer. It is possible the missing flat washer is actually stuck in the carb. Those O rings all develope a tapered shape. I usually carefully remove the the O ring in its original orientation, the flat washer, and spring, then I take the screw and lightly clean the threads on my brass wheel, or a brass hand brush. I replace the spring, washer, and O ring, and coat them with marine waterproof grease. The low and high speed screws should be able to rotate without a screw driver, just with your fingers. If not, repeat the procedure, till the screws turn smooth and easily.