Ruger mismarked the gun.
If you ask Ruger about it they will want the gun back, then you won't get that mismarked gun back.
That said, there aren't that many Mini-14 collectors but it is still worth a premium.
I would hold on to it…. It is what it is.
Ruger's letter will only tell you what they think it shipped as. Although with the serial number on the receiver along with the receiver model/caliber marking pretty well tells you the receiver is mismarked.
A letter would be further proof Ruger had no idea it was marked that way before it left (but like I said…. The letter won't verify Ruger's mistake…. They would have never shipped it if they caught the error).