Keeping the Cap On

One common problem with old 18 size Elgins, and others, is that the cap for the mainspring barrel doesn't stay on. It may seem OK, but then creep up until the bottom of the balance wheel scraps it. Here's a "fix" for this that I've never seen. Someone added an odd washer loose on the top. Of course this doesn't work real well... Tightening the edge of the barrel is a better choice.