The one on my ES2 (laydown box) was cracked so I got a new one from NOC spares - good quick service.
The last time I fitted one of these was in the early 1970s and I'd forgotten what a pain (literally) they are. Edgar Franks says "force the spring round till it engages in the 2nd slot beyond the free position".
He (or the fitter he was watching) must have had hands like a gorilla. I eventually managed it by making a loop out of thin wire (as used for lockwiring) which I hooked over the spring then putting a screwdriver through it to act like a Spanish windlass. After about the 5th attempt I was eventually successful......
The Commando was much easier.