Yeah, that pattern might not check out afterall. If you still have the boxes... I honestly dont know if the replacements come in a retail box? But I know on the retail boxes on the side of the box on a small sticker there is a "Lot #" and that we do know tells you the date. I dont know any other way on the device itself though. The way the lot # breaks down is as follows: the first two digits are the year... 13 or 14... then the next two digits are the week of the year it was produced, for example: 1352 is the last week of 2013, 1401, the first week of 2014. My current SP2 is lot 1344, meaning it was one of the first batches out the door most likely.
I don't know if the replacement units state a lot number, I would assume if they do and its a pretty recent lot then its a new device. If you're lucky enough to get a newly bumped up CPU, you can see that in the computer properties, it will show as a 1.9ghz 4300U as there is no way right now that one of those would have turned around fast enough to be shipped back out as a refurb.
I also know there's another post on this forum today of someone getting a scratched up unit. If it were me I would absolutely raise hell and send it back. The standard laws around replacement devices are that they have to be in equal or better shape (ala refurbed or new) and if refurbed can't be in worse shape than the device you're getting fixed. Laws very from place to place but I would doubt MS would fight anything too heavily, esp if the device you received has defects.
MS's warranty process is less than ideal, stock is low in retail, exchanges arent as ironed out as Apple for example where they have a stronger handle on the supply chain and channels to track returns. After processing my return online I had issues and tried to call, 2hrs and 8 transfers later I finally found actual Surface support and I must say that his ability to help me, make sense of things and get me help was heads and tails above everyone else I had talked to. If I'd started with him I would have been a 20min call and I would have been extremely satisfied. He's the one that told me that they can call me, that rather than waiting on hold for 20 mins, they can call me in 20 mins, get directly to the right person and be done. That seems like a world of win, its just not widely known which is a problem that I think is leading to more frustration between people.
I for one love my SP2, it does everything I want it to do, its an awesome machine. This coming from a long time PC guy that went Mac full force 5 years ago and is wicked impressed with the SP2, except for the hardware issues I'm having. If I didnt have those, I'd be over the moon.