Re: black watch
My dad's mate had a Black Watch. I think he got the Build It Yourself option from the pages of ETI: he was an electronics boffin in the MoD so was a dab hand with the soldering iron.
Biggest problem then was that in the 1970s shirts were often mafe of nylon. Which caused static and would fry the watch.
I remember the reviews of the Scientific Calculator in another electronic magazine of the time where the errors on the trig functions were reported to be 33%.