Actually there were differing qualities of these manufactured. I had one of the early ones and it was fantastic, I could run my fingers touching over the surface and lightly depress while dragging and the audible feedback had such low latency: joy. Sort-of regretted replacing it with mechanical keys. Later Atari 400s had barely registering keypresses that was a workout for your fingers.

I must have had a later one then.

Where did you find a replacement mechanical keyboard for the 400? I never found one. I was so envious of the Sinclair owners because they had tons of options.

I think it might have been a local creation. It was ugly as hell, half of it rose above/outside the case showing the switch bodies and entire keycaps. It did have a good feel though. I wasn't much of a touch/speed typist so didn't gain so much. I got it because my father was a bit of a typewriter geek and didn't like me using the membrane keys. Ended up getting an 800 with expansion memory but the keys weren't as good--too springy/spongey no clickiness in it's motion.