I don't think that you overpaid at all. You paid a price that needed to be paid if you wanted to own that gun. I do give Mr's Grote a lot of credit for writing a very compelling story about this K 22. Although the words "transitional model" are used quite frequently regarding the pre war to post war changes, I believe that the Seller in this case was referring to the changes that occured between 5 screw and 4 screw models. Whether or not all of those claims are accurate, (I will leave that to those with greater knowledge than me) if correct, then I certainly see an added value for both gun and box.
I personally own a pre model 14, K 38 Heavy Masterpiece that is the 1951 USRA Trophy gun and is so inscribed. While it is merely a .38 special revolver with a 6" barrel of which thousands were made, many in the collector community value these guns and I have personally seen one from a different year sell for over $5,000. So when it comes to collecting and paying for things, I really feel that it is up to the buyer and what the item is worth to them.