It fits modern S-1 Stylophone models and all of the equivalent vintage models. The pocket can hold sheet music, a patch cable, ear buds, etc. Something like a USB power supply will be too thick to allow the case to be zipped shut. (I tried it.) The case fits an S-1 perfectly, so it's too small to hold a GEN X-1.
The Stylophone S-1 Carry Case is available now. Visit the Dubreq website for more photos and details about an introductory discount.