A Sopwith Dolphin, a World War One fighter aircraft made by Sopwith Aviation, the same company that made the Sopwith Camel, was fully restored using parts scavenged from other Dolphins along with parts made to specs from original drawings. Those remanufactured parts include a pair of music-wire extension springs for the pair Lewis machine guns. The new springs were built by Lee Spring’s Europe, which is headquartered in the UK. They closely match the original set which were 2.25-in. long, made out of 20g steel wire with a 5⁄16-in. OD. Extension springs are generally loaded in tension and have hooks or loops on their ends which let them apply a pull force. Extension springs are usually attached to components at both ends, and when those components are moved apart, the spring tries to bring move them back toward each other.
The Dolphin was the first fighter plane to boast four machine guns -- two Vickers guns pointing forward through the propellers, and a pair of Lewis guns firing forward and upward, and mounted on top of the wings. While 1,778 Dolphins were built, this restored version is believed to be the only Dolphin now in existence. Restoration on the Dolphin began in 1968, and now that it is finished, the plane sits on display at the RAF Museum in Hendon, UK.