Some web sites dealing with Pentax equipment.
Bojidar Dimitrov's Pentax K-Mount Page is a very thorough and detailed source of information about every piece of Pentax K-mount hardware imaginable. The problem is that the information stops at the *ist film camera. Apparently, this site was in jepaordy of being shut down by Mr. Dimitrov, but the hue and cry of his fans has purchased a reprieve. He says he's going to start updating the information and adding new entries, but it'll likely be slow going.
Stan's Photography Site is almost exclusively reviews of Pentax lenses culled from email lists and discussion fora. LOTS of information.
Asahi Optical Historical Club has a lot of good info. I haven't spelunked it the way I have the previous links.
Larry Yaremko's Pentax Screw-Mount Page has a lot of data sheets for various Takumar lenses.