I had a 7-string Russian guitar *lowest tuned down to B* in the late sixties. Solid pine table with harder *red wood* back and sides. Beautiful rich tone. Unfortunately the neck was one piece without truss and warped badly.
I saved the body for years, but never got round to making a replacement neck *always regretted that, too*.
As I recollect there were 2 or three standard tunings for the 7-string Russian folk guitar.
Wish I still had it *wistful sigh*. LOL