I'm sorry to disagree with Paolo but there are some fake reproduced Bunds about. They fail with close inspection when compared to OTD reference pictures so this should be something you could compare and check yourself Martin. Take most note of the bezel outer surface, the camber on the shoulders of the lugs and the exposed portion of the pushers when viewed from the back.
The fact that there is not Bund or numbers marked in the back case is not so scarey. Real Heuer 1550 watches were made by the 1000's and not all of them were military issued, hence we see some genuine watches without the numbers on the caseback. Its better to see no number at all than a poor attempt to later create the case number (which we have also see come up from time to time).
The lack of reference number 1550 SG is not quite so clear. I have seen a couple of watches come up with provinence (claims to be from an original issued owner) without the 1550 SG in place. It may have had a replacement case or even a non Heuer issued case (keep in mind other producers such as Sinn used this same case without the model designation). Again i say check the size/shape of the lugs to reference photo's to see they are the right shape.
It may be worth while posting photo's of you watch here so the forum members may be able to help with their experience.
All the best - Darren
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-: Maybe they were scratched during a service. Counterfeits in Bunds
: are not so common...