US Navy NAVSUBSUPPFAC New London Medallion 6.75 x 6.75 in.
This is most likely made of either bronze or brass and at one point was likely part of a plaque. There are three small holes through the medallion for nailing it onto a plaque. It features the USN submariner dolphins logo, along with a nuceal symbol. It is just over 6.75″ across and just under 0.5″ thick.