John, it will be where DABO said it will be, below the knobby part of the
aperature ring and just above where the lens would contact the mounting
face of the camera. You have a 20mm, the 24mm and a 50mm, all of
which have serial numbers in the same place. Find where the number on any
of those lenses is and the others will in the same place.
I'm sure you are in for a big "aha" moment real soon.