Wtches are harder to "custom make" apart from maybe a modified dial, different strap, and maybe an engraving somewhere.
With your example of the Aston Martin, it's not a drastic change - if you see a DB9 on the street you recognize it's a DB9. They don't put spoilers and wings all over the place and put in James Bond style weaponry if you ask for it... or do they ?
As far as I know, it's the interior being custom designed, much like changing a strap on a PAM