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Ahh can see now, you're referring to this bit? yes?. ..and you have already worked out the cause and remedy. It's very common for parts like this to have a bronze sleave lining/bush inside and it's that part that has probably worn away causing the boss to move about on the pivot. You can fabricate a new sleeve providing you can find a similar material, I know this because taking bikes apart you come across various bodges that previous owners have tried to varying degrees of success. So the part you would need is the arm itself as the lining bush will be part of it. That's why it doesn't show up on the parts diagram. and if there wasn't a bush then the boss is just machined metal with a close tolerance when new which has simply gone oval over the years. The two options being: to either ream it out so the hole is equal all around (remove the oval) and fit a metal sleave/bushing of uniform thickness. or cut a sliver of metal and try wrapping it around the shaft, i.e bodge it. You can get thin tin brass, metal sheets in modelling shops, that you can try fabricating one from, try ebay. something like this... https://www.ebay.co.uk/itm/145190789050?itmmeta=01HYFM8YJR8XNME4P2JJBPXDRV&hash=item21ce0b9fba:g:JnIAAOSw16hkrgAq&itmprp=enc%3AAQAJAAAAwF1kt81QocyvSrSnVPobJjdkDG7uW3KM%2F4gs%2FS1p4jjAj773U51RvIl9E4q0Rw4kGb8CdrvFVuG2%2FIGtIq0MdhmdaJYIWBpQCPXcxmxvMWGO3HGbZvFP9Rq9lWvRBbJQVwjuIjL40DoIrxvZV%2Bvx%2F7Pnkj3%2FPY4I6ds4ddUWxo1C%2Btfey1YsMC0heHBRbVDaxF40Nc0uVgQYosEBcGAwmCOs3C9hUdY4KPJnbPRi%2BWWPgn4Mfk01R6oMM%2FrFXk0%2FEQ%3D%3D|tkp%3ABk9SR8Tpo_TzYw