From left to right;
H2 Store Demonstrator
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