Hi AK,
.but as far as I know, it should read "double serpent, calendar" - i.e. the double serpent means it has two serpent hands. Different form the "normal" serpent calendar, the minute hand of the "double serpent" is a serpent shaped hand as well (making double serpent hands).
To distinguish the date serpent hand from the minute hand, the date hand is polished and the minute hand remains in the original heat blued finish.
Hope that helps (and even more hope it's correct
)
Best regards
Suitbert