One of my favorite Elizabeth Cotten songs.
As for the guitar...Gibson's MK (Mark) Series consisted of only four production models - the MK-35, the MK-53, the MK-72, and the MK-81. There was one more model, the MK-99, which was special order only and hand-made by the luthier who developed the Mark Series, but no examples are known to exist of this model. So the highest model in the Mark Series production guitars was the MK-81. Only 438 of this model were ever produced, making it one of the rarest Gibson standard production models ever.
The Mark Series guitars were an attempt by Gibson's parent company, Norlin, to do something truly different - to revolutionize the way guitars were designed and built. To achieve this goal, they turned to science. In 1973, Gibson contacted Dr. Adrian Houtsma, Professor of Acoustic Physics at MIT, to confirm some research Gibson itself had be performing. Receiving positive feedback on their in-house R&D, Gibson then went to Dr. Michael Kasha, a chemical physicist working as Director of the Institute of Molecular Biophysics at Florida State University. Combining all their findings, Gibson sought the help of luthier Richard Schneider to develop Dr. Kasha's ideas on bracing and bridge shapes into a guitar which could be put into production. This was the start of the Mark Series, and it was a complete departure from all previous bracing styles. This was unlike any ladder, fan, or X-bracing that came before it. It was something truly different. And like so many of the truly different things that Gibson tried to introduce over the years, e.g. the Explorer, the Flying V, the Artist Series, and the Moderne, the Mark Series largely flopped.
It turns out, very much like today, guitar players in the 1970s were pretty resistant to change, and the Mark Series guitars did not sound or look like any of the previous flat tops Gibson ever produced. It did not help that production was marred with setbacks and sometimes shoddy workmanship as the factory at first struggled to get a grasp on how to build these new kind of guitars. But not all the Mark Series guitars suffered from bad workmanship, and not everyone who played one disliked it. In fact, Gibson did manage to sell quite a decent sum of the lower Mk-35 and Mk-53 models. It was the MK-81, with its commanding list price of over $800, which was higher than a Les Paul Custom at the time, that didn't manage to sell in respectable numbers. In fact, many of the ones that did manage to get through production were put into the hands of the salesmen who were charged with going out to the Mom and Pop shops of the day and selling these crazy new acoustic creations to conservative store owners for stock. Few succeeded in making any sales on the MK-81. The price was too high and the design too alien.
After the Mark Series was discontinued, the "Kasha Guitar" based on Michael Kasha's bracing ideas was further developed by Richard Schneider. His hand-built creations, made before he passed away in 1997, are considered some of the best and most interesting modern guitar designs, and they command vast sums of money and reside in some of the most prestigious of guitar collections. The MK-81 is probably about as close as one can get to a Schneider guitar without it being made by his hands. Other luthiers, some of them Schneider proteges, have taken up the mantle of building in the Kasha style.