The Westley Richards blog says "the distinctive action-back shape of this rifle indicates the maker was probably Daniel Leonard".
Leonard's, gunmakers in Birmingham, England made a number of N.E. Double Rifles "for the Trade" including W.J. Jeffery & A. Hollis - I have identified 21 N.E. Double Rifles that are "Leonard-made" for W.J. Jeffery (14 guns) & Hollis (7 guns). An Leonard-made Jeffery .600 N.E. Double Rifle was sold by Rock Island Auction in 2007 for US$43k.
Rusty Knight (R.I.P. 2022) & "400NitroExpress" (R.I.P.) and R. Nolan Keiler did a lot of research on the Leonard-made N.E. Double Rifles. They identified that a Leonard-made Double Rifle had a 3 or 4 digit Makers Number stamped on the barrels under-rib just forward of the fore-end lug & the same number also stamped on the action underneath the bottom action plate. If it was a "finished" gun then that number was also stamped in ink on wood on the interior of the stock where it joins the action & under the forearm metal. Harry Leonard was a son of Daniel Leonard. The firm started in 1832, changed its name to Daniel Leonard & Sons in 1866, and ceased trading in 1966 after 134 years of gunmaking on the death of 3rd generation Charles Leonard (no sons).