I was thinking about the same as Dons.
The rear bridge is the problem with this one and there looks to be something going on with the barrel just ahead of the receiver ring. Of course, the enlarged photos don't show it.
Double square bridge, solid left wall though - very rare action.

How about the .505 at Champlin's?

http://www.champlinarms.com/Default.aspx...&GunID=1430

Somebody buy this gun before it causes damage to my marital bliss!!
Another DSB, solid wall action, but magnum length!
Looks like a real deal to me.