In a lovely bit of observational selection bias I read this

one of the company’s main expenses was postage: each programmer would write their code on carbon paper (backup copies were needed in case the original was lost or damaged) and send it through the mail to their client’s punchgirls. The code would be transcribed, the program run and any errors would get posted back to the programmer for correction

in the, well worth a read, article A Woman’s Place about a group of freelance women programmer’s in the 1960s/70s/80s yesterday, and then this morning saw this Inside the UK’s last carbon paper factory, a lovely video showing the machinery and people working at York Haven, the last one-time carbon manufacturer in the UK.

It had never occurred to me that carbon paper would have been used as a backup for coding, but why not, it was an available “technology”, and programs in the 60s were hand written before being made into punched cards.