I also say SNK made a couple thousand of them. If anything these must have been over-produced, for like anything else SNK made after having over-anticipated the demand, these things were eventually sold off new, or used, with great discounts. Even into the mid 90's one could get a used one for peanuts (less than what a typical joystick now goes for).
Don't bother with what Super Sellers charges. I saw the controllers at a couple other shops in Japan that wouldn't ship overseas, and for $1000 theoretically one could have had 20~30 of them, as long as the order could have be filled.
Pornographical mahjong games seem a staple of the gaming industry in Japan and while there is only truly one adult hentai mj game on the Neo (CD; Final Romance 2), the others are all themed around the same subject. Minasan is kind of an exception.