Ahhh, I just got back from Summer vacation. I missed some fun here!
@ErichPryde is correct in noticing the missing ridges on the underside of the endlinks-- you were indeed sold a fake.
It's easy to research the fakes that are being sold online-- all the "replica" SM300 Heritages all happen to have the ridge-less same endlink and are mated incorrectly with the 20mm STZ001159 clasp. You're the second person in the last 2 months that I've talked to on here that got scammed by an eBay seller (which is quick to accept a return to protect his reputation to continue scamming) with multiple fake bracelets being sold at real prices, as well as other fake straps/bracelets from brands like AP and Patek.
Who'd you get it from on eBay? There's one guy from Germany and another from Israel that is known to be doing this en masse.
Fake SM300 Heritage with the same smooth endlink as yours. Same GIJ factory code on the bracelet and HJE on clasp:
The STZ001159, though irrelevant because it's not the correct clasp for this bracelet, is easy to spot as fake since the factories in China haven't adjusted their fonts yet. I think the very early genuine models had the "9" in STZ001159 with the curved tail (don't quote me on this), but every genuine example I've seen for years has the straight "9" tail.
Fake (look at the 9 and the bad Omega anti-counterfeit logo touching the triangle):
Genuine:
AVOID EBAY FOR OMEGA BRACELETS/STRAPS LIKE THE PLAGUE!!