Per reddit:

Narcotic Revenue Stamps were used from early 1900s (Harrison Narcotic Act) to sometime around the 1970s when the DEA was created (Controlled Substance Act). Before the DEA, narcotics officers were part of the IRS/Treasury department. The stamp was used to denote “legal” narcotics and served to help track transactions. Prescribers also registered with the IRS much like they currently do with the DEA. Instead of a DEA permit, they displayed their registration number with the IRS. They fee that paid and their registration number was noted on a similar type of special tax stamp.