SR 34 and SR 8A have not been on the books for a long time, as they were both eliminated in the 1976 renumbering. Most of the remaining signs, as far as I'm aware, are located out on the far reaches of the northwest corner of the state, except for the circle 34 signs in Gerlach. So, to answer the question, the signs were simply forgotten--and are probably some of the few examples of pre-1976 Nevada numbering left in the wild.
In regards to the picture, I believe this to be a sign from after the renumbering. This sign, located just north of Gerlach, is the junction of former SR 34 & SR 81. In the renumbering, new State Route 447 was assigned to old SR 34 south of Gerlach and old SR 81 north of Gerlach. I believe portion of SR 447 north of Gerlach (old SR 81) was relinquished to county control sometime in the 1980s. My guess would be that the sign was posted by the county, and that's how the county decided to sign the old route (standard county pentagon shields are posted at the real end of current SR 447 in Gerlach--probably an NDOT installation). Washoe County maintains some of the old state highways in this area, retaining the pre-1976 state highway numbers--signage standards vary, but they appear to be somewhat consistent in their own adaptation of NDOT-inspired mileposts.