Morning PT.
Firstly I assume you are referring to the clubsandy.com site. This is not my site, I just work with them. The credit for this site lies with Sandy and Andrew Youngman.
The majority of locations on the site are places in Budapest and a lot of people use the same locations over there so that's why you'll see the same places cropping up time and time again. It's also why we are shooting a lot of sets for the site(s) in places other than Budapest.
I'm afraid I know nothing of the German productions. As far as I know they have no link to clubsandy.com or 21Sextury which is the company that clubsandy is a part of.
I don't know the Martins personally but I suspect they are involved with the company that runs Silvia Saint's official site (www.clubsilviasaint.net) and Adriana Malkova's official site (www.absoluteadriana.com). Again I work with these models shooting material for their sites and there's a guy I deal with called Martin but I've never actually met him.
I don't know of the Absolute Stars business but I'd be interested to check them out!