Kyrie,
I over simplified a bit in my analysis I suppose, and I'm sorry about that. My point was that when the A and D boxlock came along in England, it took a bit for the market to sort out which would be the best. To that end, gunmakers did indeed finish boxlocks to the same level as sidelocks, and a few, priced their boxlocks actually higher than their sidelock guns-Greener, for example, with the Facile Princepts.
I'll let others debate what makes either design "best" but, one can find English boxlocks from the era I am speaking of that have chopper lump barrels, dropper points in the stock, church windows, a scalloped action back, and intercepting sears. The engraving and finish would have been equal or superior to the same companies sidelock model.
It seems to me that by the time the A and D patents had expired, and the Spanish invoked the sincerest form of flattery, the market in England had decided which version was the best, and would thus be the most expensive gun they produced, or, the only design they would produce.
And the Spanish took those lessons, and ran.
Now, a question, if I may?
Was there ever a time when a boxlock was AYAs, or some other gun makers, most expensive gun to buy in Spain? That did happen in England. Was there ever a Spanish boxlock that included all the features I named above, and sold for more money than the companies top of the line sidelock, to the best of your knowledge?

Best,
Ted

PS Not to be too critical, but, the gold inlays on the gun you posted are not to my liking, not to mention whatever is going on with the floorplate not fitting up. Gamescene is a difficult task master, and few humans have ever mastered that art form. Just my opinion.
I do like Winston Churchill's inlay work. I won't be buying any of it, any time soon, however.