A Google search for "John Calvert Leeds, England gunsmith 1865" brings up results including the first two entries from gun auctions below:
1, "John & William Calvert were English Gun makers with various premises in Leeds between 1797 & 1865 including 73 Briggate, Lowerhead Row & Commercial Street (see page 186 of British Gunmakers Volume 2 by Brown)."--->>>
https://www.gunstar.co.uk/john-will...ussion-overcoat-pisto/Pistol-Hand-Guns/991385
2. The same quote is mentioned for another english gun auction here:--->>>
https://www.bidspotter.eu/en-gb/auc...1136/lot-9a6c46a0-a35e-4cb9-9f9c-a7fa01029dde
3. Another Google search for "John Calvert Leeds, England gunsmith Confederate revolver" brings up an auction for a cased CSA revolver with photos:--->>>
http://www.vincaponihistoricantique...co-marked-bowie-knife-all-identified-to-benj/
A SCARCE CIVIL WAR CONFEDERATE ENGLISH MADE "J. CALVERT" MARKED PERCUSSION REVOLVER IN ITS ORIGINAL CASE WITH ACCESSORIES & "CORSAN DENTON BERDEKIN CO." MARKED BOWIE KNIFE ALL IDENTIFIED TO "BENJAMIN KIBBLE OF THE 3RD TENN. INFANTRY" IN NICE UNTOUCHED CONDITION!
A SCARCE CIVIL WAR CONFEDERATE ENGLISH MADE "J. CALVERT" MARKED PERCUSSION REVOLVER IN ITS ORIGINAL CASE WITH ACCESSORIES & "CORSAN DENTON BERDEKIN CO." MARKED BOWIE KNIFE ALL IDENTIFIED TO "BENJAMIN KIBBLE OF THE 3RD TENN. INFANTRY" IN NICE UNTOUCHED CONDITION. ALL ITEMS ARE ALL ORIGINAL & COMPLETE. THE REVOLVER HAS NICE METAL, MARKINGS, & ENGRAVING ON THE FRAME. IT IS MARKED ON THE BARREL "J. CALVERT, LEEDS. THE BUTT OF THE GUN IS MARKED "KIBBLE, CSA"....."