Boxlock and sidelock are the pointing and flushing dogs of SxS shotguns. Each has its merits, and it’s hard to say if one’s superior to the other.
To see what sets these guns apart, check out this page: The difference between a Boxlock and a Sidelock.
It’s from Boxall & Edmiston.
A decade ago or so, I met two Englishman at The Southern SxS: Peter Boxall, formerly a manufacturing director of Holland & Holland, and James Edmiston, former MD of the Sterling Armament Company. They had just launched a new shotgun company — Boxall & Edmiston — and their future looked bright.
Unfortunately, things didn’t work out. But as of today, their website is still up and it’s filled with tons of info about guns and gunmaking. You can find it if you go here Shotgun Anatomy tab.