The Cogswell & Harrison Avant Tout Ejector hammer gun's previous sales, I found sitting right there on-line:

2002: https://www.christies.com/lotfinder/Lot/...75-details.aspx

AN UNUSUAL 12-BORE 'THE AVANT TOUT' HAMMER EJECTOR GUN BY COGSWELL & HARRISON, NO. 14742
The action incorporating an unusual ejecting mechanism set behind the forend-loop, toplever, percussion fences, rebounding backlocks, best foliate-scroll engraving, highly-figured stock (cracked at the head) with horn butt-plate, the barrels with game-rib
Weight 6lb. 8˝oz., 13 7/8in. pull, 30in. barrels, proof exemption.

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Sold to the Gun Broker dealer, August 29, 2020:

https://www.proxibid.com/Firearms/Black-...mation/55932594