The 400 if you acidentally stepped on it it'd flip back and bark your shin, you'd fall over and put your foot through it. that's why there's so few today.
I had a solar-powered pocket calculator with a membrane once. Credit-card-sized. You can tell I was a nerd because I owned three pocket calculators at the same time.
Yes, but the 400 had it on a spillproof membrane like you'd see on a fast-food cash register. The later models improved the layout but most had much worse switches than the 800.