How can it be a 577/500 when the groove size is .463?

From what I see it's probably a 500/450 Number 1 Express
2 3/4" case. But I'd re-do the chamber cast and heat the barrels a tad warmer.. I think that's the problem.

It's ironic that there is another Turner listed on Gunbroker now. As nice as yours maybe.

Edit; Here's the twin.

http://www.gunbroker.com/Auction/ViewItem.asp?Item=105149721