Brevet, which means patent, was engraved on early production runs of certain models that contain patent-pending features. In addition to the case engraving, the crown would also contain the standard brevet +.
The earliest 6542s would have the brevet marks while normal production (most) did not.
I have a few Acetimers that have worked flawlessly for several years. They are quiet, operate on either A/C or batteries, have multiple speeds (to suit different watches' winding requirements) and look great
Rolex, by far. Excellent fit/finish with sufficient spring tension to keep the pins extended into the lugs, which has been a problem with all aftermarket 'bars in my experience (including all mentioned above).
The parts that are visible look gen, but the dial was repainted at some point. If you are considering a purchase, I would ask seller to send good, clear macros of the movement & engravings between each of the lugs. That will give you a better idea of what it is.
Avoid reps with "gold" (it fades or rubs off), "jewels" (always look fake) or alot of complications (features that do not quite 'look' right).
The more details on a rep, the more likely it looks like a rep; the simpler (fewer features), the more likely it will pass as genuine (gen). & read Toad's guide.
Other than gens, I have never seen rep/aftermarket 118138 straps available. Assuming you do not want to buy the gen strap, why not just pick up a cheap rep DD & just use the strap?