Generally speaking British #5 shot ran about 220/oz or a near exact equivelent of US #6. I have absolutely no idea however if "Derby" shot of that day had their own scale or not.
I would really, Really, love to se a photograph of the pattern that Purdey threw at 112 yds with a 1 1/8oz load of that shot, & to also know how many sheets of a "Petit" pad it penertrated.
MeThinks perhaps some "Servant" was trying to get in "MeLord's" good Graces by exagerating his prowess.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra