@gelehu: I'm sure one "could" fit the interrupter with a different pattern of notches for the rotating gear that handles ringing,
Those would be cam wheels. There is one for lamp flash, one for lamp wink and of course one for ringing. Each cam wheel has a (sort of microswitch) switch that has an arm with a roller that follows the contour of cam. When the motor starts all cam wheels, being on the same shaft, rotate which causes the switches to turn on and off in step with indentations on their respective cam wheel. The speed of the motor and the spacing of the indentations on the cam wheels are what determine the lamp flash, wink and ringing patterns. Simple enough.
Now, if you wanted to generate a European cadence, all you would have to do is make a new ringing cam wheel with indentations that would generate that ring pattern. That assumes you could get the interrupter apart to replace it.
And of course this is very old school. With electronics this could be done very easily. I vaguely remember somebody who actually made and sold a solid-state interrupter replacement? Maybe this could be your next project- with a switch on top of it to select US or European ring cadence.
-Hal
