-- The top sample is a known fake (dark blue, fishbone pattern, now marked Heuer), and we see smaller numbers.

-- The bottom sample is the subject of our discussion, the soon-to-be-famous "Bullitt Silverstone", and we see larger numbers.

Why are these done in such different styles? Impossible for me to believe either (a) that Heuer had two different batches of these cases made (which would explain the different styles) . . . the production would seem to be too low, to have required multiple batches of these cases, or (b) that these could have come from the same batch, but look so different.

My conclusion: private stamping, done by different people, or maybe the same person, at different times.