I have an IBM microswitch (yep, clickity clickity click) keyboard manufactured by lexmark. It is probably one of the first keyboards with a ps/2 connector, it was old when I bought it in `99 for a dollar. I love it, and it is so heavy I could use it as a weapon (but I would trade it for a microswitch keyboard with more keys!)
But besides its other charms, it seems to have been printed with diamond ink. It has at least 7 years of heavy use on it, and was probably someone’s office computer keyboard when it was new. F2 is missing, but other than that, the letters do not show any signs of wear. Lesser keyboards, that I did not like anyway, have begun having ghostly or difficult to read lettering after less than a year. What did lexmark use to print these durable letters?