My search to identify the tattooed man in a 1901 French police archive photograph - a tale of daggers through the heart, 19th-century milk advertisements, and a French paediatrician with a sideline in tattoo collecting.
A more academic account of 19th-century tattoo collecting, museum archives, and the history of criminological interpretations of European tattoos can be found here:
Captain Peter Johnston-Saint was employed as a purchasing agent by Sir Henry Wellcome from ????-1936. He traveled all over Europe looking for objects of interest for Wellcome's 'Historical Medical Museum'.