There are 3 basic variants that I am aware of - Standard S20 that has a fixed rear sight and a 'sporter' style grip.
S20 Match that has sporter grip and a adjustable rear sight. S20 Match Export that has the adjustable rear sight and a target style grip with a flared grip with a thumb rest.
They can be tricky to strip, so unless it does not work I would take the action out of the grip and grease and oil the spring. You could also stand the action on it's end and pour some neats foot oil down the barrel and leave it to soak into the leather washer for a few days. Some models had a 'oil' hole on the chamber.
Some good info here:http://www.muzzle.de/N3/Druckluft/BSF_S_20/bsf_s_20.
html[IMG]DSC01115 by Mark Davis, on Flickr[/IMG]
DSC01108 by Mark Davis, on Flickr