Balogh: Excuse me because I don't mean to be a smartass, but ONLY in Switzerland (the land of beer keg bolt handles, diagonal sight dovetails, and cuckoo clocks) would such an intricate extrapolation be suggested. (Actually I find it endearing because the CH is the last holdout in the world proudly maintaining such national idiosyncrasies.)If a date code (e.g. 69) is visible next to the proof on the frame, then you can easily calculate the year of the proof by adding 1900.
Haha... I'd say twice on target and once just missed. Surprisingly, the cuckoo clocks haven't their origin in Switzerland. They are not manufactured there either. It's the region of the Black Forest in the South of Germany which is famous for these... okay, let's them call clocks....Switzerland (the land of beer keg bolt handles, diagonal sight dovetails, and cuckoo clocks)