Just noticed your post and realize that my response is a little dated. Hope you have found a solution by now. In my experience these are seldom available unless you run across someone who happens to part out a Whitney Kennedy. The spring from either a large frame or small frame rifle or carbine will...

The rifle is probably a large frame Whitney Kennedy with a 28" barrel. It could possibly be a Whitney Burgess, but the cartridges in his belt loops do not appear to be 45-70, which the Whitney Burgess would have shot. They look more like 45-60.

I'm no expert on the subject, but some of this reasoning comes from the model 1878 Burgess rifle (The first lever capable of handling the 45-70 cartridge). The gun experienced documented instances of rounds going off prematurely during cycling operations due to the sensitive primers used in one part...

Thanks for the support guys! Got no horse in this race...just an interested lever gun collector who likes Winchesters, Marlins and Whitneys.

For those of you who may have a specific interest in Whitney Kennedy, Whitney Scharf, Whitney Burgess, or Burgess Shotguns, Evans and Bullard guns, there is a newly established Whitney Burgess forum. If you have an interest check it out at: http://whitney-burgess.com/