From my ledger pages-

SN 22111 - SN 22120 = Build dates October 1910 - November 1910
This was a page full of Leonard-built 12-bores including a very cool sounding 'paradox pattern' set up for 28gr or 34gr Cordite and a Nickel Base 750gr bullet in the 2-1/2" case, weight 7#, 10oz!

Notes I've taken from other conversations noting the Jeffery serial number sequencing have another block of SN (24241-24480) from 1913-1923
That would suggest your rifle was made in 1923 or 1924
A look at the proofs would be helpful.

SNs 25581 to 25590 (yet another ledger page, from another NE member) includes one of his rifles, built in 1939

I'll keep digging, but this gets you close!

Cheers
Tinker