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The submarine version of the Japanese 20 x 120 mm binoculars were composed of an entirely different construction than the preceding 20x designs . The model is encased in a thick jacket of steel and was used as a deck mounted observation binocular on submarines. The binocular itself weighs 170 pounds with 40mm thick optical windows that protect the objective lenses, while the eyepiece end is equipped with a water-tight door that is sealed by tightening a hand wheeler. Thick glass and steel protect the instrument from sea pressure. IPD is adjusted by turning a lever located between the eyepieces which in turn works a geared system that rotates the prism housings. In essence, these optics are scaled-up versions of the 80mm instruments with Porro II prism systems and Erfle variation eyepieces with 60 degree apparent field angles.

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