Rusty Knight (RIP 2022) & Mark Cash (RIP 2019) and R Nolan Keiler did a lot of research on Jeffery N.E. Double Rifles. They identified that a number of the Jeffery N.E. Double Rifles (and A. Hollis N.E.Double Rifles) were actually made by the gunmaker Daniel Leonard & Sons of Birmingham.
Those Leonard-made N.E. Double Rifles have a 3 or 4 digit Makers Number stamped on the under-rib just forward of the fore-end lug & that same Makers Number also stamped on the Action underneath the bottom action plate. If it was a "finished" gun then the Makers Number was also stamped in ink on the interior of the Stock where it joins the action and also stamped in ink under the forearm metal.
Chris