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@gbj_tester
It's an eccentric (for chain tensioning), of a design which was quite popular on custom/aftermarket (Harris, Spondon, Metmachex etc) sports motorcycle swing arms back in the day, before stock ones were good.
The advantage over a slot fork end and tugs is better control of the axle position under loads which twist the wheel about the vertical axis, which is a big problem on bikes which are being flicked from 45°R to 45°L lean angle in a fraction of a second while their 10kg wheel/tyre/sprocket assembly is rotating at >1500rpm. On a bicycle, not so much.
The disadvantage is the short range of adjustment, so you can't change your gearing much before you have to cut the chain.
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