As to how the windows were made, it was usually a punch. There's a negative of the shape cut through a steel cylinder that goes inside the lug socket, precisely aligned with the positive punch that shears the steel of the lug. I have seen hand-operated, screw-powered punches, but those are slow, so larger-volume makes usually had punches that could be whammed through rapidly. I used one that was foot-powered, punched out our windows in less time than it takes to say "wham". We had a wider window for the DT and a narrower one for the TT and ST lug. The ST also got the wide window punched in back, below the seatpost pinchbolt.
It took something like one minute total to make all 4 of these windows. Wham wham wham wham!
There were also lugs you could buy with the windows in them already, like Prugnat S-4, but using those makes your frame look generic, exact same windows as everyone else. So many builders had their own punches made, to differentiate the product, give them some individuality.
Oh right you can also drill one or more holes and then shape them with needle files. At R+E in the early '80s every lug got a trillium flower (3 petals) made with 3 drilled holes and a triangular needle file. We got reasonably fast at that, but it sure wasn't a Wham! Hand-filed windows are very rare due to the time it takes. Even Hetchins' curlicues were stamped out with a punch press.