Production numbers...where do they come from?

[Chevy454]

Well, just like the other thread I started, this topic came up on the Pure Stock board.

Where do the production stats for certain body/engine combos come from? I mean, how do we know there are 4,475 LS-6 hardtops, but aren't 100% certain how many were convertibles? Did they come from Tonowanda? GM? I had never really thought about it, but you hear a lot of "1 of xxx made", and I would like to know how they arrived at certain numbers.