You don't see 'em very often, but a Stemac Robbiati model has been offered on New Zealand's TradeMe site with a black steamer knob. The machine looks a little bit tired around the badge, but otherwise very tidy, and the relative rarity of the knob should make it interesting for a collector.
At a rough guess, I would say that 98% of all Atomics that change hands have the classical deep red steamer knob. Early Sassoon models often had light blue or light red knobs (these were slightly smaller in diameter than the "standard" model). The American La Sorrentina (the original ones!) sometimes had an emerald green, but these are fairly rare. Black knobs and yellow knobs are also out there, also fairly rare.
