I have one of the original Nikon 50mm F/1.8 AF lenses with the hard plastic focus ring. But can't find the serial number.
If anyone knows please let me know.
I remember reading somewhere that the numbers are located on the lower part of the aperture ring (below the knurling) 180 degrees from the largest aperture. So, somewhere on the opposite side of the ring. Finely engraved and hard to see.