Just seen this video through google searching all kinds of variations of spring assisted underlevers, I haven't really gotten on to fully restoring this gun yet as i have a load of other projects on at the moment, but i have disassembled it a few times and there has to be a reason for this coil spring inside the mechanism. The spring and parts are obviously worn but when you catch the spring just right you can feel the spring tension on the under lever. I guess I'll just have to work it out when i get to it. But if anyone comes across any diagrams in their reading please let me know!
