This is a Mitsubishi Electric silicon MOSFET 30MHz, 16w power transistor. It was designed for HF RF power amplifiers and was used specifically in the Cobra 200GTL as the final transistor (Cobra part number 010133). RD16HHF1 transistors are typically advertised as RoHS compliant, but ours may not be as they lack the "G" after the lot marking (as pictured above).
MOSFET conductors are sensitive to ESD, so please use caution when handling and installing them. Use of a grounding strap when handling them is strongly recommended.
More information is available on the PDF data sheet linked below.