Makes you wonder if Greener was a lefty, doesn't it Dustin?
There are English boxlocks that have intercepting sears. There aren't many of them, however. And not all of those feature Best Gun levels of fitting. But, some do. As to the screw in hinge pin being a feature of a Best Gun, I have my doubts. It is a feature that offers up little in the way of superiority vs any other way of accomplishing a knuckle joint on a double. It isn't like a Best Gun comes with a spare fitted hinge pin in the case designed to be serviced in the field-guns usually wear on the lump. Hell, my Tobin has a screw in hinge pin, it can't be that tough to do.
Going strictly by OWD's description, couldn't a graded, American, Baker be considered a Best Gun?
That being said, I fall pretty much in agreement with Greener-the boxlock is an improvement over the sidelock in shotgun design. The sidelock was nothing more than a hammergun, with the hammers moved inside. The king shot a sidelock. Some of his hanger's on still shot hammer guns. You likely didn't. But, you could get a boxlock, finished up as well as a sidelock, with all of the same important features. It would cost about the same as a sidelock.
Fewer were made at this level of fitting then sidelocks, but, that doesn't mean they weren't made or they don't exist.


Best,
Ted