When the McSpadden's owned and operated McSpadden Dulcimers, the solid soundboard was offered as an option. When Jim Woods purchased McSpadden Dulcimers the solid soundboard became a standard feature on all their dulcimers. I don't know the exact dates, but there would be two dates:
Date One - McSpaddens offered the solid soundboard option for the first time.
Date Two - Jim Woods made the solid soundboard a standard feature of all McSpadden dulcimers.
Jim Woods purchased McSpadden dulcimers in 2001, so the switch to the solid soundboard on all instruments would have occurred sometime around 2001-2002.
updated by @greg-gunner: 08/11/22 01:21:18PM