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one of my holdsworths has a 5 digit number, the other 6 digits but that began with a zero. if it's got a transfer badge on the headtube then i think these appeared from the 80's onwards.
incidentally i didn;t know that norman kilgariff who gathered loads of info on grubb and holdsworths passed away last year. thats a loss to the cycling world.
i would prob say that earlier holdsworths would have 4 digit number and i believe that it was always stamped on the fork steerer too.
I guess you are right about the forks.
The lugs look like prugnat 62s.
They start using them in the 70's I read, but the brazed on derailleur clamp indicates its from the 80's no?