This stick was bundled as standard equipment for all SNK home cartidge systems. It uses arcade switches for the buttons, but uses custom buttons without the commonly used piston/sleeve assembly common to arcade buttons.
The buttons are custom, but the stick is (almost) a standard part, nearly identical to those used in sticks from other manufacturers (like Micomsoft, ASCII, Sigma). There's More details on the mech on NFGcontrols.com.
It has two outputs for the D button, but apparently only one is monitored by the console.