Probably the best dedicated vintage airguns site is the American
"Vintage Airguns" forum. It has various Falke-related threads going back several years.
http://www.vintageairgun.com
A fast up-and-coming vintage airguns forum, also with some threads on the subject of Falkes, is the more recent Collectables section of the UK-based
http://www.airgunbbs.com.
Another decent UK airgun site with a Collecting section, although as yet it has only one or two Falke-related threads, is
http://www.airgunforum.net
In Germany the Co2air site has some Falke content.
http://www.co2air.de
Also in New Zealand, this forum has some interesting Falke threads:
http://www.nzairgunners.com
One site that has discussed Falke underlevers in connection with Hakim air rifles is Tom Gaylord's blog at
http://www.pyramydair.com/blog/