Tuning Air Rifle Haedquarters FWB 124 Spring Installation?

Normally the bushing goes inside the piston and the washer could be added at either end depending on firing behavior. The washers are generally used to fine tune the firing behavior after new spring and tophat are installed. Make sure the ends of the spring are smooth and polished so it can rotate freely.

The long spacer should go in the piston. The other end should have a steel washer that slips over the guide, and the spring then goes onto the guide. 

Ive used a few of those kits over the years. They work well. 

Be mindful of making sure the seal isn't too tight. Lots of those guns had some really tight tubes and the seal needs to be turned down quite a bit. Otherwise power output will be low, and temp sensitivity high.