From Deutsch, this metal panel nut is for use with the HDP20 range of connectors.

HDP20 Range

A range of Heavy Duty thermoplastic, multipole, circular, sealed connectors.,Available in 2 sizes ranging from 8 to 47 ways, in standard or `reverse sex` options.,Straight or right angled clam shell backshells accepting a variety of conduit sizes are also available. Uses Deutsch solid contacts size 12, 16 and 20 depending on the planform selected. For suitable contacts see for example, size 12 (stock no. 425-828), size 16 (stock no. 425-761), size 20 (stock no. 425-834). For a suitable crimp tool see stock no. 425-973.

Sealed to IP67
Durable – 100 mating cycles minimum
Vibration proof – 20g's at 10 - 2000Hz
Corrosion Resistant – 48hours Salt Spray to MIL STD 1344