For the parts you need, you might have better luck with egun.de.
http://www.egun.de/market/index.php
Or a specialist German retailer.
eg.http://www.waffencenter-gotha.de/sho...5_46_1311.html
I can't help you on your question, but I did once own a FWB 603, and I still have a FWB 300s.
I could be wrong but my memory tells me that the 603 was heavier. But that could have been partly due to the laminated stock.
Post this on the Classic Airguns and Target sections of this forum and somebody should be able to give you a definitive answer.