WPC machines used a washerhead style #6 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.
These #8 screws are used in just a few applications, mostly ramps to mounting posts. They are useful to have around.
Here's a picture showing the two heads side-by-side http://iobium.com/truss.JPG
Bag of 15 screws ( 5 ea 3/8 and 5 ea 1/2 and 5 ea 1/4)