"I believe there was an ink stamping thing, a bit like the ones used for stamping dates onto paper, which was used for this. See, for example,
https://www.vintassion.com/vintage-6...r-labels/I3923 "
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The machine in the photographs in the above link show a 'Machine No 31' used for stamping the numbers on dial labels for use in kiosks. The larger numbers were stamped on blank Labels No 160 that had been printed with the exchange name in large letters. That machine was introduced circa 1960 when the GPO started putting a large 'X' on the end of kiosk numbers to indicate to the operator it was a public kiosk. It can clearly be seen after the 6th digit in the above link photos
The original used on dial labels prior to 1960 was a 'Machine Numbering No 5' - see the Swanage label in attached photo.
Circa 1960 it was replaced by a 'Machine Numbering No 32' with similar sized numbers but of a slightly different font. See the Amlwch label in the attached. Note the different style of the figure '3'
Ian
CNet 0352 2345
MR ETD 053-6278