It's really difficult to say how common it was, but besides the few surviving photos of individuals who had done this, there are contemporaneous reports of tatt

It's really difficult to say how common it was, but besides the few surviving photos of individuals who had done this, there are contemporaneous reports of tattoo artists seeing a significant increase in business after the passage of the social security act. In newspaper archives, we can find references to this practice fairly frequently in the 1937-1940 time period, including headlines like "social security law boon to tattoo artists" and some fun ones like "victim is identified by social security tattoo." Of course I'm sure the practice was relatively fringe, but newspaper archives show us that it was not, on the other hand, especially isolated. Newspapers report on the practice occurring locally in nearly every state (that existed at the time). I also think you somewhat underestimate the popularity of tattoos at the time. While they were regarded as fringe and somewhat antisocial, they were widely available and particularly popular in some circles, as they are today.