WPC machines used a washerhead style screw for many posts and ramps that is very hard to find. They are also often chewed up. These stainless steel trusshead screws are similar in appearance; the large diameter of the trusshead looks just perfect on post tops. WPC-95 machines switched to hex head screws for this application.
Here's a picture showing the two heads side-by-side http://iobium.com/truss.JPG
Bag of 15 screws ( 5 ea 1/4 and 5 ea 1/2 and 5 ea 3/8)