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.