What does Dr after some person's of firm's name on a invoice mean It is not doctor It is on many 19th century invoices both pre-printed forms and handwritten slips of paper?

In the context of 19th-century invoices, "Dr" often stands for "Debtor," indicating the person or firm that owes money for goods or services rendered. This abbreviation was commonly used in accounting and billing practices of the time to denote outstanding balances. It is not related to the medical title "Doctor."