Chris,
The C4 type ships were World War II-era cargo vessels, built predominantly by Henry Kaiser's shipyards as well as Sun Shipbuilding and Dry Dock in Chester, PA, where
Marine Robin was constructed. 75 C4s were built, of which only ten were to the C4-S-A4 specification (general cargo). They were 520' in length.
They're also tricky little buggers to research.
This page contains a list of all C4 vessels, with a photo of one. I also able to scrounge up
this model of a C4. I'm guessing that
this is the photo you've already seen of
Marine Robin. This particular vessel appears to have been fitted with a set of derricks forward in addition to added gun tubs, likely wartime alterations. One would have to dig around in the Navy's archives to find the actual number of modifications. Unfortunately, these were unglamorous cargo ships, so each vessel might not have been extensively photographed. C4s were also refit more notably as
Haven class hospital ships for the Navy during the war.
In regards to
Thompson, she is a unique vessel. I saw her from afar last summer.
Josh.