I don't have a Steringworth, Thorny. I missed the Fox boat by being involved with slidey breech guns.
I think a Fox is pretty, but, never ponied up for one. Now, like I said, if there was ever a gun that should crack, in my opinion, it would be one of Frank Major Tobins-the central action body is cut to hell making room for the lockwork, under the lockplates, and that big assed lump, between the tubes. They are often, and I mean real often, found in an off face condition, with little or no wear on the hinge pin and lump. The action bar, you see, having been bent, prolly after being stuffed with near anti aircraft duck loads.
However, since in the case of either a Fox, or a Tobin, the real culprit is most likely abuse, the real question must be why do the catastrophic abuse photos so often depict a Fox?
Hey, I missed the boat on Spanish sidelocks as well. To be honest, my few experiences with true sidelocks were always waiting for repairs, or waiting for the next incident of needing repair. Empire guns were boxlocks for a reason, 'Matey. My Tobin is simply a relic, and not a good example of either boxlock, or sidelock design.
And, nobody on my street has one.

Best,
Ted