Moin Jürgen.

Vielleicht hilft Dir folgender Absatz aus dem Lange-1-Guide:

"[...]Lange decided to hide such an exceptional view from the eyes behind a thick layer of gold in 1994. Because, during the ’90s, it was not the norm to showcase movements. Lange introduced the sapphire caseback versions only a year after under different reference numbers; while still continuing the production of solid caseback Lange 1 versions too."

Demnach kamen die Sichtböden ab 1995.