As Denny suggested, the fast idle link from throttle to choke identifies the carburetor as the 684s. The 684s was the last of the production W-1 carburetors, and, as such, incorporated ALL improvements, modifications, etc. and, all things equal, would be the very best of the W-1 carbs.
Unfortunately, all things are not equal, and since the 684s design was only used one year, many parts are less available than the other models, and some are quite expensive.
I personally do NOT like the spring on the top of the pump, but just personal opinion.
Jon.
