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Explanation: Here is what the tool looks like: (link below)
I'm not much into motorcycles (seems this is a motorcycle thing) but it looks like this is some additional cutting (Handfräsen?) of valve seats. Presumably, the precision of the cutting is ensured by a rod of soft metal whose diametre provides a close (albeit not tight) fit into the valve guide channel. I can't figure the process clearly, though... hope there are some motorbike freaks (or ex-freaks ;) who'd lend a hand here.