Yeah, I'm pretty sure they do use the same drivers as the SR-X Mk3, but they're more comfy due to the comfier pads and they use the acoustic lens. In other words, SR-5 with SR-X drivers. I don't know why they're so rare here; I think there are a bunch in Europe. The one I picked up was on Ebay BIN for a week or so before I bit, so it's not like anyone was looking, either.
Edit:
http://cgi.ebay.de/STAX-SR-5-Gold-Set_W0QQitemZ290314571438QQcmdZViewItemQQptZKopfh