Thanks to this article on Typotheque
I learned something new the other day.
Old computer keyboards used the same style of lettering for the characters on the keys... as one sees in blueprints and other material produced by drafting. So, naturally, I figured that a LEROY lettering guide from Keuffel and Esser was used, being a resident of North America.
But from a recent visit to Martin Wichary's Twitter, I heard the name of "Gorton". What struck me immediately was that the numerals reminded me of the ones I'd seen in Nixie tubes.
Eventually, searching for more information led me to the Typotheque page... from which I learned that instead of being an original American invention, the mechanism used in the Leroy lettering guides first existed in its developed form under the brand name Standardgraph, and the principle was earlier used in something called the Polynorm.