Bought this for a wallet busting $3.00 at goodwill. Definitely linear. The outer spring is responsible for the the switch return. With the keycap off I have to manually pull up on the slider to get it to return. The action isn't too smooth, but not gritty. Feels heavier than a mx black but haven't taken any measurements.
Full pictures here.
http://s195.photobucket.com/user/luckyduck_man/slideshow/calculator
