I bought a cross-hair dome dial, dogleg Constellation at auction a few years back and it came with box, papers, receipt, hangtag ....and an Omega fix-o-flex bracelet with 558 end links.
(it was purchased from a NAAFI, likely somewhere in the Far East, by an RAF chief technician.
So, as it's quite a practical option, I suspect it was bought with the watch.
I can't say that the end links are an exact fit radius-wise but they kind of work (better on the wrist than in the photo, as the links more-or-less adopt the line of the cranked lugs.)
BTW, I took the bracelet off straight away when I got the watch serviced and wear it on a leather strap
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