A very rare 1970’s Mathey-Tissot Skin Diver - Ref. 1631 - powered by a “Ultra High Frequency” (UHF) ETA 2784 automatic movement. (f ~ 28,800 A/h vs. 21,600 A/h). Reference 1631 came with various iridescent dials including blue, red and green, as well as being available in black. There were both male and female versions available. These are tough to find, especially in nice condition, but a gem if you do!