HD, good questions and good points. I suspect that a great deal of the interest in O/Us in other places and the US was a result of post WWII economics and advertising. It was after the war that the SP took off. I started trap shooting in the mid 60's and took it up again in the early 70's. At that time the SP was the only mass market O/U with any real presence. The P guns had just started to appear and there was the occaisonal Ljutic, SO Beretta and Kreighoff. The SPs were considered expensive and the others off the charts. Other than the SO most people perceived the Berettas, correctly or not, as "cheap" Italian guns. The P guns did a lot to change this. Field guns were much the same and the SP was still considered a premium gun. Almost no one even knew what a Woodward or a Boss was. In the 70's the SP went south and Beretta started to make inroads. Browning brought out the Citoris and a number of good Japanese and Italian guns followed. Winchester jumped in a little later with the 101s. All were heavily advertised and the darlings of the shooting rags. Prices were affordable and the economy made them moreso. The SxS was all but forgotten by all but a few. I'm sure that it was much the same in western Europe.
The change cane in the mid 80's. We were out of the recession, there was lots of money around and some goods developed status. This is when the Rolex, European cars, etc. became symbols of sucess. About the same time the price of vintage US guns started to take off closely followed by English guns. The appreciation of dynamics in a field gun took even more time. Though many would hate to admit it, most of the lower grade US vintage doubles were pretty clubby, hence the interest in the Fox, LC, and Parker O frame 16's.
O/Us may dominate the mid to higher end market but the SxSs are obviously making a comeback. It may not be the golden age but it is certainly the rennaisance of the SxS. The choices available today are the broadest since before WWII.
