If you have the actual manufacturing date, you can maybe find out by matching it with iOS releases. Usually the batch of devices manufactured a week or so before an iOS would most likely come with that particular iOS, aka the latest. But that's not really the case all the time but generally what happens. But if you could selectively pick the device based on manufacturing date, do a bit of research on the iOS release date and try to get a device manufactured around the release date of the iOS.
If you get an understanding rep or talk your way through it, the rep can probably tell you what iOS is on the device when they look up the shipment/manufacturing date