McGuinn's signature model Martin acoustic 7-stringer is a fairly new issue, and would only be heard on recent recordings (if at all), not on any of the more famous Byrds-era stuff, which featured his electric 12-string (a Rickenbacher?) pretty much exclusively.

Spider John's 7-string guitar seems to have been the prototype for McGuinn's. Does anyone know if he invented this configuration, or picked up the idea from some earlier musician? Whether or not he rigged the thing himself? What make and model guitar he might have modified? Anything? Anyone?

I never much noticed or appreciated Steve Masakowski's 7th low string in the context of Astral Project, a four- or five-piece combo at various times in its history*. But a year or so ago, I witnessed a duet set featuring Steve and a piano, and it was very clear that he was able to supply a "bass" part throughout, while at the same time producing sounds more readily recognizable as (lead and/or rhythm) "guitar" parts.

* The lineup originally included David Torkanowsky on piano, along with Steve on guitar, Tony DiGradi on sax, and the incredible rhythm section of bassist James Singleton and drummer Johnny Vidacovich. They've been together for something like 30 years, since they were the best young contemporary jazz players in New Orleans, a kind of all-star aggregation. (Now they're the best middle-aged players.) Dave dropped out of the group just a few years ago ~ amicably, as far as I know. Piano and guitar fill pretty much the same role in that kind of jazz ensemble, and as time went by and everyone kept growing and deleloping, those two instruments seemed to increasingly "crowd each other out."