When the 3940 was introduced in 1986, some customers wanted to have a display case. At that time, Patek did not offer a choice with a 3940, so they came up with a new reference number, 3941, to apply to all and only those 3940‘s which had a display back. This move proved popular and four years later, the 3940 was offered with both a solid and display case, and so the 3941 reference number was retired. I read there are only about 35 or so 3941‘s that were made. It’s a rarity — I think some collectors are thrilled by the difference, whereas others don’t care about it.