Sorry to say some people that put down a lot of time to replicate items does for some unfathonable reason not make the parts exactly as original. Here's some examples of original stamps and bossings on shells and lights for 1930 and -31. Between 30 and 31 there can be some subtle difference in the height of the hump above the bezel, but I can't say for sure, it can be difference in manufacturing, presstools or such..
But...repro many times can be found with more than acceptable quality and finish and will fulfill it's task with honours until you find a usable original item...