L.Brown,
The 12/20 was patented by William Baker, in my opinion the most inventive gunmaker ever, and offered by various firms, including Stephen Grant, under different model names. The location of proving is no guarantee of the location of manufacture. Note that no maker credits the inventor with a small stamp for this action or other Baker patents used.
Brittany Man,
The perception that the monobloc line is a matter of subjective taste. If the issue is not to see the joint (reminds me of the comedians Morecamb and Wise lines re a wig) then all other jointing methods would outrank the monobloc as would laser welding on the monobloc itself. While true demibloc (as in the Ideal, Winchester Mod 21, BSA) can technically stand comparison with the monobloc, the others with the convoluted mating surfaces held only by braze do not. True demibloc is silver soldered by the way, not brazed. Considering that barrels are the most "consumable" part of a shotgun the monobloc is the only system that makes provision for this eventuality. Personally I would rather do a simple tube change than sleeving or rebarreling.
As to the original question re chronology and quality. Over the years I have dismantled many shotguns. There is no visible quality difference that can be based on date of manufacture. However, there are differences even among guns by the same maker, mostly on the wood to metal fitting. So far the best overall internal work I have ever seen inside a SXS was a pinless hand detachable sidelock by Fabbio Zanotti made in the 1980s.
Re the fit and finish issue. In some English sidelocks I have seen the impressions of lock screw slots in the wood. Now, those were definitely NOT chiselled out after smoke fitting. A stockmaker friend insists that they are made by soaking the wood in linseed oil and pressing the locks onto the wood for a kind of forced fitting. In his opinion this is not a kosher way to fit locks. All the guns I have seen with this trait are prewar.
