what would you do?

A V12 is marked V12. V12I is marked with an I and a V12R is marked with an R. How did you and/or your friend not know for a year and a half? No one looked at the serial numbers or the faceplate? It's probably too late to do anything now, but maybe you could get them to spring for an upgrade to a V12R. They're a very nice amp, but run very hot. I'm just not sure how the consumer and the seller both dropped the ball. Best of luck.

"what would you do?"

I can tell what I wouldn't do, which is to start three threads on the same subject.

Beyond that, you need to provide more info as Tpreaves suggests. How is this the wrong amp? Is it simply a matter of the wrong serial number being transcribed on the receipt? Or two amps of the same model being misboxed?

And if the same model amp is involved, how is your friend being harmed by this? Has the dealer refused warranty service or otherwise refused to address any issue beyond the serial number? Is the amp new or used?

No one can give provide any meaningful advice until you provide more information.

Checks should have been done as soon as the amp was received. I don't think your friend has a leg to stand on since it has been over 1 year. Some high volume dealers may mix stuff up but most of the time if they deal in high volume they have good inventory control.

Also as long as they got a Cary v12i amp and not a totally different amp then what is wrong if the serial numbers are different? Is each Cary amp customized or is this amp a collectors item where having a lower number from the run matters? I'm just confused as to how the serial numbers would really matter in this situation.

Your friend could try to talk to Upscale Audio but even then what can they do? The other Cary v12i with the serial # might have been sold already. It has also been over a year.

I think your friend may just have to enjoy the amp he has.

Why did it take you a year and a half to figure out the purchase was wrong, and why come on pinging a dealer without asking him first?

Sorry but just a little confused. Who buys amps based on a serial number? Every piece of equipment that I have bought new was by a model number; not serial number. If the carton was sealed, good enough for me. The only time I could see this being important is if you were buying a used amp and there were two identical ones out of their boxes that you auditioned. You liked one better than the other, picked it, and the serial number was the only way to differentiate between the two. Was this the case? Was it a used purchase? I cannot imagine in a new purchase a serial number would matter as long as you got the right model you were purchasing. Am I wrong? If so, can someone tell me why?