Citizen was the first company in the world to make a watch case fully from titanium. Although common these days, manufacturing challenges made it difficult to work with in the early 1970s. Citizen made and sold less than 2000 of this X8 model, all of which I believe were produced in May 1970. Using the 0820 movement, it is an Officially Certified Chronometer measured against the Japan Chronometer Association’s standards at the time. The 99.6% titanium case is a dull grey, not polished like many watches typically seen today:

You can find more info on this model here: https://sweep-hand.org/2013/12/22/this-weeks-featured-watch-55-the-x8-titanium-chronometer