Patek Philippe

The original 3940 had a solid case back.  And a variant of the original 3940, called the 3941, came with a sapphire crystal case back.  This only lasted a couple of years before Patek sold the 3940 with both types of case backs due to the popularity of the sapphire case back.  And the 3941 designation was effectively rendered obsolete.

Here are two more concise descriptions of the differences (I hope I'm not getting into trouble already by quoting auction house info):

Christie's description:

Reference 3941 is possibly the rarest variant of the 3940 family of automatic perpetual calendar wristwatches. Since the glazed backs were not numbered, Patek Philippe chose instead to apply the case number inside the upper case part, underneath the bezel.

Manufacture of references 3940 and 3941 was launched in 1985, the difference between the two reference numbers being their backs:
- ref. 3940: plain snap on back
- ref. 3941: glazed snap on back

To respond to their customers increasing demand for the two backs, Patek Philippe produced the model as of around 1988 with both the plain and the glazed snap on back. Production of reference 3941 was discontinued in 1990.

Antiquorum's description:

Ref. 3940 and 3941 When Patek Philippe first launched this new series of watches in 1985, Ref. 3940 had a metallic clip-on back, while Ref. 3941 was equipped exclusively with a sapphire crystal back. The reference number of this model was soon standardized to Ref. 3940, and was sold with two case backs, except for the platinum model. Consequently, very few examples bear the Ref. 3941.

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