When a roll of film arrived at the photofinishing shop, a return envelope was created. Data such as film type and size, size of prints desired and the surface (glossy - matte, simi-glass), number of prints per negative, plus name and address of customer. The bag was stamped using an ink numbering devise.

The number on the bag corresponds to an adhesive label called a twin-check. A twin-check with this same number is applied to the undeveloped roll of film. A duplicate twin check accompanies the roll into the film developing darkroom. In total darkness, the roll of film is unwound, the duplicate twin-check is applied to its tail end. The film is then developed in a processing machine. Next the developed film is taken to a printing room. The film is now printed by projecting an image of each frame onto light sensitive photo paper. As each frame is printed, an inked numbering device called a back-printer, prints an identifying number on the back of each to-be-developed print paper. The number corresponds to the number on the twin check.

After film processing, printing, print developing, film, prints and envelopes arrive at an assembly station. In the envelope goes developed film, prints, and the envelope is priced. This numbering scheme allowed correct assembly of the finished goods. The envelope is now ready for pick-up -- A twin-check - back=printing numbering system similar to the system used in a laundry.