First chance I've had to time this vintage Deville. C. 560. Last serviced Nov 2009.
Since its not hack able I had to start the watch up after wind down as close as possible to the atomic time.

Start 9/29.
Watch - 8:24:00a
Atomic - 8:23:57a
difference of +3 seconds to atomic.

End 10/6.
Watch - 8:15:00a
Atomic - 8:14:45a
difference of +15 atomic time

Total net variance +12 seconds over 7 days.
Daily variance +1.7 seconds a day.

How's that for a 50+ y/o non COSC watch? Outstanding!