While long barrelled, my 1876 10-bore Westley Richards double has as mentioned no standing leaf, so may as described in Marrakai's article, being designed for quick shooting.
John - Forsyth wrote of this type of sighting (all leaves down) for DR's, in that the enlarged breech & rib height, gave ample elevation to the ball for point blank shooting. This will certainly be fun testing in such a beautiful rifle.