Pictured above is an original example of an A-13450-B2 tail lamp lens. I have several other very nice lenses that may be original, but I am not certain on some of them.
Some have 12 flutes and some have 11 flutes. Most (all?) of the reproductions I have seen have 11 flutes, not 12, as well as having weak red and amber colors.
The original Ford detail drawing A-13450-B2 dated 12-6-30 (researched at the Benson Ford Research Center) specifies the lens as having 12 flutes. It is also identified as "Ford Design #2"
The earlier releases of A-13450-B2 dated 3-20-29 and 7-10-29 also have 12 flutes and the correct stipple pattern on the bulb side of the lens.
The following series of pics show a comparison of original and reproduction tail lamp lenses.
Shown above is a comparison between an original 12 flute lens on the left which matches the A-13450-B2 drawing specifications, a typical 11 flute reproduction lens in the middle, and a questionable 11 flute lens shown on the right.
Note that the original 12 flute lens on the left has the characteristic deep ruby red color, whereas the typical reproduction 11 flute reproduction in the middle has a very pale washed out appearance. The questionable lens on the right basically has the correct color appearance, but only has 11 flutes, and has a different inside texture.
Viewed from the inside, note the textures of both the stop and tail lamp portions on the original 12 flute lens on the left, as well as that of the reproduction in the center. They are quite similar. The questionable lens on the right has some other style of textures.
Pictured above is a better direct comparison of the original 12 flute lens on top and the questionable 11 flute lens on the bottom, showing the different textures.
The stipple pattern texture on the bulb side of the (upper) original lens matches the drawing requirements exactly. The pattern on the (lower) suspect lens does not match the drawing.