That is sketchy. I wouldn't accept a wheelset that supposedly had DT rims that didn't come with the vinyl decal. Decals are definitely valuable if you're ever trying to resell them and 350 hubs laced to either rim could easily outlast your bike frame.
Worth noting that the 391 is $107 a rim and the 392 is $57 a rim. Those prices are listed on DT's website and I've never had an issue with a local shop being able to match that price.
I'm sure some Sun Ringle MTX 33 rims laced to DT Swiss 350s would feel better than the stock wheels but it's still weird. I recently bought a used DT Swiss 1501 rear wheel for $80. It had been on Craigslist for nearly a month. It's got a 28 hole 240S straight pull hub with the 22.5 mm 1501 rim. I would have been a bit more concerned about its history if the rim was unmarked. Just letting you know as cautionary advice.
Haven't been to the LBS yet, but after checking the rims online, or what i could find (there is some sparse info around):
Basically the rims are unmarked because they are sold by DT Swiss to bike brands to make their own wheels.
Specwise they are 23 mm internally and 27 mm external. 28 spokes per wheel.
Also did a longer ride, and i really loved the feel of the wheels.
Longer chattery sections werent as harsh on the hands as the stock wheels.
But that might come down to tire pressure aswell since the rim is a tiny bit wider so i could drop pressure a bit.
Furthermore i could really feel that the wheels responded a lot better, with acceleration or when cornering.
Overall: great set of wheels for the price and the upgrade i was looking for.
Lesson: for the next wheelset: be specific about what i want and do more research beforehand. Instead of leaving things "open".