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I've searched online a lot, and found plenty of sites that quote both numbers.
There appears to be agreement that the C-mount flange focal distance (FFD) is 17.526 mm (exactly 0.69 inches).
The CS-mount is either 5.0 mm less or 5.026 mm less.
Does anybody have a definitive answer?
CS mount is 5mm less than C mount. However, there is no such thing as "exactly", and a 0.026mm difference isn't significant as long as lens elements can move far enough to focus.
