That's a handsome constellation you have there! Unfortunately, I think the store's response is correct. The very first OEM leather strap I'm aware of for the integrated-bracelet Connie design debuted with the 2013 Sedna constellation pie-pan dial (reference 123.53.38.21.02.001)
Interchangeable OEM leather straps weren't introduced for the Constellation line until the very recent Connie refresh a couple years ago. I don't think the leather straps from either the current generation, or the 2013 limited edition Pie Pan, will fit the older 4-digit models.
You could ask a bespoke watch strap maker to see if they'd be willing to make you one? I anticipate they'll want to see pictures or videos of the end-link of the bracelet, and get a paper tracing of the bracelet taper, so they know how to cut the strap. This is one of the reasons I came very, very, very close (almost swiped the credit card inside the omega boutique) to purchasing that 2013 Sedna Pie Pan, but ultimately decided against it because I wasn't confident Omega will keep their promise of supplying OEM straps for 30 years after a model's introduction.
