Just had a look at Sanwa Denshi's japanese site.
http://www.sanwa-denshi.co.jp/
Their line of pistol-grip controls consists of M11, MX-3G, MX-3S, MX-A and Blazer. No mention of the MX-3 FHSS.
My best guess is the MX-3 FHSS was either never sold or has stopped selling in Japan.