Currently I have 9 Asturias guitars including a JM20S and a JM15C. (I still wait for the coupon that says: Buy 9, get 1 for free.)
As you probably have found out by yourself already, Asturias is a guitar manufacture (or workshop) in Japan where a few skilled luthiers build guitars since about 1980. The history of Asturias is linked to the two key luthiers Masaru Matano (who founded the company, and already built guitars in the 70ies), and Tsuji Wataru. More information about their history can be found in the internet.
Asturias has changed the names of their models a few times. At the time the JM15 was built there were basically 4 models: A8, A10, JM15, JM20, where JM stands for John Mills, and the suffix S for a spruce top and C for a cedar top. The guitarist John Mills played an Asturias guitar, and the company named their two top models accordingly. After the connection with the guitarist broke up, they renamed their models to: Standard, Prelude, Custom, and Renaissance, that means the JM15 model is not built anymore. In the internet you can read that a JM15 is now named Custom, but the models are not exactly the same and have changed over the time. For example, the purflings of the JM15 and the Custom are different.
The JM15S has a solid spruce top, solid rosewood back and sides, and and ebony fingerboard. In my view the quality of Asturias guitars is excellent. Just take the two "entry" models Asturias Standard and Bernabe M5 that are both sold at a similar price level, and then compare the quality of the wood and the build quality. For me an Asturias Standard looks more like a luthier-built guitar, and a Bernabe M5 like a factory-built guitar. (Beside my 9 Asturias guitars I have seen and played various Bernabe Mx guitars. This is my experience and other forum members might of course prefer Bernabe guitars.)
Just let me know if you need more information...
Happiness is when what you think what you say and what you do are in harmony.