Gaugedrifles,

Lefever in partnership with other gunmakers (first Ellis, then Barber and finally Nichols) did conversions of percussion to breech loaders, hammerguns to hammerless, and made some double rifles and combination guns in addition to his double shotguns. I'm not aware he ever sleeved shotguns with rifle barrels (could this have been done in the 1870-1890 era?)

If the 5764 is a serial number that Lefever provided, that would date the gun to the partnership of Nichols and Lefever (1876-1879).

Jeff