proximo

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

I have one I got "free" on promotion when I bought a Walther PDP back in March. I see no problem with its brightness in full sunlight and it has a larger window compared to the Holosun 507c x2 I also have. Elevation and windage adjustments are firm with positive clicks. The glass is clear and bright. You can find them for under $200 online. I like it. I've shot somewhere around 500 rounds with it mounted on the PDP and its still ticking and holding zero. if nothing else it's cheap enough to keep in your range bag as a backup.

The first link in the original post to the Trigger Pull Chart is dead. I believe this is the correct one: http://www.glockmeister.com/EvaluationofTriggerPulls.asp