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Di Modell Rallye is the default choice for virtually everyone looking for a factory-look rally strap. Nicer than Omega's own offering imo.
Cheap to buy, last forever, look good, work good (though I have small wrists to need to cut the 'leash' that limits the travel of the second keeper so that I can move it further along the strap toward the watch.
Actually wearing one of mine today, and on a Speedmaster too.
ETA:
Has the advantage of having two prongs, so the holes for fastening the strap carry on the rally style pattern all the way.
Easy to find nice looking straps with Google, hard to discern the quality of said straps, hence asking for a recommendation which I would hope would be based on direct experience.