Before email was popular and mechanical typewriters weren't just for hipsters, the quickest way to make copies of a document was to use a special sheet of paper called carbon paper.
Lawyers imitate what previous lawyers have done. After all, a good deal of roped legal work, such as drawing up contracts, can be copied in large chunks from one document to another.
He soon suspects that BSI is laundering money for a long list of the international high and mighty and surreptitiously copies of documents which he tries to bring to the attention of the financial services authority.