Jerry,
Jeff Dunbar and as well William "Mike" Dunbar, reportedly his brother, who might have been linked to the Winnamucca holdup, are mentioned briefly in Ernst (2009), Burton (2012). Patterson (1998), etc.

Jeff Dunbar seems to have first made his mark, using the word "mark" loosely, in 1887, when he was "acquitted of the charge of shooting" in Sheridan, Wyoming, though fined for carrying "a concealed deadly weapon." He upped the ante in 1892, when he killed a man in a bar room brawl in Casper, Wyoming. He was tried for murder & acquitted.

In 1894, he stole $15 in a Silverton, Colorado, faro game, and relieved at gun point a fellow sporting man of $200 at "Mid's Hell," a Fort Duchesne, Utah, gambling den. (His accomplice in the latter crime, one William Hughes, was killed the following year in yet another bar room fight.)

In the mid-1890s, Dunbar was said to be terrorizing the citizens of Dixon, Wyoming. (Will Kane must have been out of town.) Both Jeff and his brother were in the saloon business there. From the tone of the articles, Jeff Dunbar was one mean drunk.

In 1897, the same year he marries and settles in Dixon, Jeff Dunbar's name surfaces in the national press, Boston Globe, November 5, 1897, as an associate of Butch Cassidy. The story is accompanied by a sketch an Alfred E. Neuman looking Butch Cassidy. From there on out, Dunbar enjoys a brief national notoriety as a mass murderer and leader of a gang of upwards of 500 outlaws.

In July 1898, a drunk and ornery Dunbar is shot dead in a bar room fight in Dixon.

Best I can tell: Notches on Dunbar's gun, 1; total take from his crimes, $215.

Jack Stroud undoubtedly can tell us more, as well as correct the errors in my account. Dan

PS Most of the above info from the usual digitized newspaper vaults.