I have a Hollis/Leonard double rifle. Leonard is well established as maker of Jeffrey doubles (perhaps not all of them, but a significant number). On the double rifles, Leonard put a 4 digit code on the underside of the barrels. Not sure about the shotguns. My boxlock is straight sided, but a friend's near identical Hollis/Leonard double rifle has the scalloped receiver.
I also have a scan of a page of Jeffrey records showing Hollis as maker of a series of double rifles.
Thanks,
That adds Hollis to the list, along with D Leonard & Sons as makers of double rifles for Jeffrey.
The citation I've found lists H Leonard & Co as making shotguns for Jeffrey.
Regards
Ken