I originally put up this page with very little information on this spot besides it being rumored to have once been a bait & tackle shop. After speaking to my uncle and grandpa (both whom have lived here for over 60 years) and other old timers, I was finally able to get an answer!
The structure once housed an electric box for a light at the end of the jetty that served to warn boats of the jetty's location. It also helped guide them in as this area can get quite foggy at night. Before this structure was put in, there were at least a handful of accidents and capsized boats.
The structure was eventually replaced by the solar light that is there now. Today all that remains is a skeletal foundation, filled with graffiti scrawlings. We used to fish off this jetty when I was a child but sadly I do not remember seeing this structure. We brought my dog the day we visited, which I regret as this is not an easy hike and I would not say it's dog-friendly.