a 16mm film reel only has the spiral on one side and would be 20mm on the outside.

the last one, 12mm on the outside measurement, would be for minox film, which is 9mm wide. for 16 and 9 mm wide film you clip the film to the center and then feed the film in at an angle so it is held in place by the single spiral.

fyi -- minox reels are fairly hard to find, usually priced accordingly.