laundry mark

noun1

1. •
   (historical) An identifying marking, usually in coded form and handwritten in indelible ink on an inconspicuous area of a garment, widely used until the mid-20th century as an organizing aid by merchants who cleaned and pressed clothing.

   • One of his shirt cuffs . . . bore a laundry mark, and the detectives finally located the laundry in this city. — 1888