That pin is the upper link arm pin. There are two. The left side pin should be solid and the right side is hollow. They are a friction fit on the 550B (they are threaded on the 550C).
This Dillon video on doing a major cleaning and lube of the 550B shows how they are removed and re-installed. In the first photo you link to, it doesn't appear the pin was hammered in fully the last time the press was broken down for cleaning and lube. Just from the gunk on the pin, I'm guessing a good cleaning and lube is long overdue