In my various readings, I seem to recall that lights were ordered by size and flash. Those two parameters would have been set by the appropriate lighthouse authority, making each light made to order. Another point of trivia that has stuck with me comes from a visit to Ken Black's museum in Maine: not only were the lenses all made to order, each one was so unique that each screw was hand tapped. You have to put the screw back into the exact hole that it came from for the threads to match. Talk about a potential nightmare if you managed to mix 'em up!