The Colt House revolver, better known as the Cloverleaf (and sometimes as the Jim Fisk Model) was the first revolver Colt designed from the ground up for rimfire ammunition. It entered the market in 1871, and was only made for about 5 years. It's colloquial name came from its unusual 4-chamber, .41 caliber cylinder.
The Jim Fisk moniker came when the larger-than-life robber baron "Jublilee Jim" Fisk was murdered by Ned Stokes with one of these revolvers over a love triangle gone bad. You can read about the events and the man at Murder By Gaslight, or pick up H.W. Brands' book for more detail.