Metallic hermetic package.
This mixer is a variant of TFM-15 with six pins instead of four, two pins further enhance the ground connection.
TFM-15-9 is a very interesting mixer, it is triple balanced, also called double double-balanced mixer.
Triple balance mixer employs two diode quads in a monolithic circuit to obtain superior matching in a push-pull configuration, the schematic diagram is more complicated then simple double balanced mixer.
The main benefit is to obtain a wider bandwidth and good isolation, the LO level and IP3 are improved by 3 dB.