I’ve got a Falcon Black Diamond, had it since I was a teenager in the late 70s. Mine is a 1978 model (I've proved that from a 1978 catalogue). I’ve stripped it down for respray and renovation, and found date stamps on the back of the Weinmann callipers. Check those on yours, it should help narrow the date down.
I’ve been posting update on my progress on my blog
http://falconrestorebike.wordpress.com/