But in the first few years, the display casebook was not offered on the 3940 per se; it was reserved for the same watch but with the different reference number 3941. After 1990, Patek dropped the reference number and made the 3940 with a display caseback, too. Also there can be some difference in the case maker; I understand some different makers were used who could engineer for the display casebook in 1986-1990. After that, it was standardized.