I guess the British usage of yellowboy, referring to a gold coin, was known in the US as well. In Huckleberry Finn (1884) a character reacts to a pile of gold coins by saying, "Look at all them yallerboys!"
The "Gold certificate" USA paper money did in fact have a yellow tinted back engraving. This made quick identification easy since other bills were known as "Greenbacks" or in slang as Frogskins.
The US Dollar bill became known as a "Buck" because one dollar was the agreed upon trade value of a deer skin at one time.
Yellow Boy was also the nickname for the 1866 Winchester lever action rifle. That model fired the older Henry .44 rimfire cartridge so a brass frame was considered sufficiently strong. The later 1873 Winchester fired the more powerful .44-40 centerfire cartridge so with that model they went with a stronger case hardened iron or steel frame. Other than the metal used for the frame these rifles were nearly identical in appearance.
This might be of interest to anyone illustrating a old west story in comic form or planing a spaghetti western film.
Also if writing a late 1860's post civil war old west adventure using the Yellow Boy carbine would be suitable where using the 1873 would not be. Also the Colt Peacemaker came along in 1873 as well so and 1860's western would require either cap and ball revolvers or C&B pistols converted to use brass cartridges.
Also the commonly seen "Buscadero" gunbelt was not invented till long after the time period most western stories are set.
Such details can make or break the readers enjoyment of such stories.
