According to Mason, Mason was paired with a Mark Shearman, West was accompanied by Bob Chalk and they were to make their way through Iowa to Kansas and thence to Texas. West shot and killed a man, eluded capture but his commission was found in his hat and thus let Mason and Shearman be treated as POW's and sent to Alton. As the story goes...

There is a TB Shearman mentioned by Mudd in "with Porter" as well as an attendee list of a UCV event in 1902. Listed as Co G, Majors Brigade, Porter's Command.

There is a Jeff Chalk listed as prisoner Alton Ill, died in captivity, of Co H Coleman's Cav.

West remains elusive even today. Only Francis Marion West I find with any service record was a Private in several EMM, PEMM, and MSM units.
So perhaps the thread is in 1864, West was part of a recruiting effort to support Price, and due to circumstances unknown was separated from main body and was working to effect an escape back to Price in Kansas via Iowa when accosted by local authorities. Plausible but primary documentation is scarce.