From the images, almost a zero chance of it being a sideplated boxlock. It is a bar action sidelock; count the which of what pins and pegs you fancy. It has the Purdey nose and side frame reinforcement but no sideclips. Interesting Sheffield Steel Works stamp. I do not think the tubes were sourced from England, but an ingot or bar stock may have been imported and the tubes made in Liege.

Kind Regards,

Raimey
rse