Mechanical stopwatches are no longer in the production program of most watch manufacturers. Therefore, most of the stopwatches offered in various portals (eBay, etc.) are older used goods, or old stocks.
The statements "functional, running, all functions faultless,...." only say, that the movement is still moving.
An accuracy is not given with most of these offered watches, because due to resin, spring weakness or wear and tear, such an accuracy cannot be achieved even by adjustment attempts. These watches require at least a complete cleaning, which is associated with costs of EUR 50.00 to EUR 300.00 depending on the movement and brand.
They should also be brand name watches. Most watchmakers will wave goodbye to any exotic watches.
Mechanical stopwatches are subject to different rules than electronic or quartz watches.
A direct comparison is always to the disadvantage of the mechanical watch, even if the movement is really perfect.
Nonsense for the rally use of mechanical stopwatches:
Watches that are permanently mounted on holding plates and cannot be removed do not make much sense, because in practice they cannot be read while the car is moving.
Watches with different dials (30/60, 100/60, 100/30) only confuse and overwhelm your co-driver, but they are not really useful for a rally, but rather serve more as a visual eye-catcher in your classic car.
Clock mounts with central release are only useful for time keeping during lap drives. In this case, only crown stop watches can be used, because stop watches with a lateral reset or drag indicator watches make the central release ad absurdum. For lap time measurements, it should be possible to switch at least 3 watches simultaneously > 1 starts - 1 stops - 1 is reset.
Clipboards with mounted watches are not necessarily nonsensical but impractical. Turning the pages of the roadbook becomes a problem.
Not to forget with clipboards made of metal, is the risk of injury for your co-pilot during abrupt braking maneuvers.