When people here say "Quietkey" they're referring to old beige keyboards, which the SK-8000 is. I had a beige 105-key Dell rubberdome that was pretty decent – no idea on the model number as it wore out and died. That one had carbon-coated domes, probably over an actual PCB instead of membranes, as I took it apart once to clean it as part of an increasingly futile attempt to keep the one good keyboard I'd ever had, working. (OK, blue ALPS notwithstanding.)
The slimline sculpted models include the SK-8115 ("midnight grey") and L100 (jet black). I find both prone to emitting a horrid rattling noise, and while the SK-8115 is light but gritty, the L100 keyboard is stiff and gooey. Neither keyboard is worth bothering with, and the KB1421 is a significant improvement all round.