From left to right;

H2 Silver
H2 Store Demonstrator
H2 Black

Being in North America, the "H2"s are much easier for me to find than the "S2". If you look hard enough, you can often find them at a garage sale, in craigslist etc.

Pentax H2H2 Store DemonstratorAsahi Pentax H2Takumar 55Takumar 55mmH2

From Asahi Pentax - S / H Series