Upon review, yes there was a third, decommed VA 158 (briefly mentioned on both the
VA 56 and
VA 151 entries). It was created in 1947, when US 29 was moved to its present-day alignment between Amherst and Colleen. It followed the former alignment of VA 151 west of Shady Lane to US 60 (VA 151 was thus rerouted along old US 29 to the present-day US 29/VA 151 intersection), then the former US 29 alignment east of Shady Lane/VA 151 to Colleen. You can see maps of the change under the
VA 150 #1 entry.
In 1954, the stretch west of VA 151 was dropped, becoming today's SR 778.
1970 is when the rest of VA 158 (between VA 151 and US 29) was renumbered to VA 56. I'm not sure when that USGS Quadrangle was created, but if it was around 1970 (entirely plausible), that would explain why it shows both 56 and 158. And Microsoft conveniently fed on that for the Bing map. As far as I can tell (and looking at the CTB minutes), there was never a 56/158 duplex...it simply switched from one route number to the other.