With a little patience, I found the following information on the GC82S on Yamaha's Japan site (translated online...but understandably not all that well):
"GC 82 S" is a model pursuing a new sound with reference to the sound produced by Spanish producer Santos Hernandez and German producer Hauser I and features a rich bass and a bright high tone, and outstanding distantness Is realized.
Pursuing beautiful sounds good sound quality
In addition to beautiful sounds, we have made various improvements to gain rich volume and rise.
The top board has studied the optimum thickness, and it is now possible to obtain a bright sound with a good volume and a good rise. We are also designing a new Hibiki arrangement so that the bass sounds well from bass to treble. In addition, painting on the lower piece and the back of the neck has made the paint film thinner than the conventional one, so that it transmits rich vibration energy to the entire guitar, improving playability. Furthermore, by improving the machining accuracy of the lower piece pillow and groove, the loss of vibration transmission was reduced and the sound rise improved. Both are due to our accumulation of research unique to our company and skill transfer and improvement from skilled craftsmen.
Simple and elegant design while inheriting tradition
Hibiki Hole and Shimomekawa have inherited the traditional inlaid symbols, and made a bright image design."
""GC 82 S" and "GC 82 C" were added mainly to the "bonding and accuracy of woodwork", "the thickness of the front plate", "the length and shape of the rhinestone stuck on the back of the front plate", "painting ". In guitar making, all processes influence the tone from selection of materials, but the tone will change not only depending on how it is made but also by the person who makes it. Along with the trend of the times, new materials, adhesives, paints, etc. are born and mechanized, but Yamaha continues to keep the traditional recipe."
Also from the October 2016 catalogue:
"To get rich sound volume and rising, in addition to the beautiful timbre piled up various improvements. Table plate research optimal thickness and, get the large volume and the rise of bright sound. From the bass to the treble balance sounds good, arranged the Symphony bars newly designed. Also painting of the bottom piece and neck back to approximately half the thickness of the coating by transfer the entire guitar a rich vibration energy, improved playability. Improved precision under the stone pillow and grooves to reduce vibration transmission loss and the rise of sound is improved.
Using the GC82S Custom order products ¥ 1150000 (excluding tax) hard case with the finest in Germany pine veneer (front plate), Madagascar rose single plate (side plates). Gc79 and bass, S. Hernandez, Hauser I... pursued new sound, referring to the sounds of the world. Rich bass, treble, far of brighter extends bei model was. String length: 650 mm front: Germany pine single plate back and side panels: Madagascar rose single plate neck: Sedro fingerboard: ebony finger Board width: on pieces of 52.5 mm torso joints of 62.0 mm bottom piece: Madagascar rose yarn: 35G510QC-M treble strings: Augustin Regal bass strings: Augustine Blue label painted full shellac and paint Accessories: hard case"
If I understand the somewhat clumsy and obscure translation, the design of the GC-71 was updated by the original designer (from another document: naniki-san: Mr. Natsuki?) with changes to the soundboard thickness, top bracing, shellac finish thickness (back & neck) and the accuracy of the lower piece (back?) pillow & groove (tongue & groove?) fitting. The results are claimed as an improvement in volume & sustain, projection, bass resonance and evenness, treble brightness/clarity. I cannot make sense of, "the length and shape of the rhinestone stuck on the back of the front plate"? Can anyone clarify this phrase and the "stone"..."pillow & groove"?
I also noted that Yamaha adjusts the GC82S for Augustine Regal trebles/Augustine Blue basses...so the setup is for high tension strings. I'm not sure how the Augustine tensions compare to others such as D'Addario?
1971 Yamaha GC-10