You can speculate as much as you want and call it logic, but in the Rolex world, there is a long history of fakes getting better and better as fakers iterated to address the highly publicized tells. It seems that you think it's trivial to study every detail, but apparently it's not. Details are elusive. Ultimately experts learned to keep some things to themselves, I know I have a few things that I look for on a Rolex clasp and I would never post them. I guarantee that the same thing has happened with Speedmaster bezels. Many of us are just sitting on some things we look for, and we're not eager to post them.
Maybe you have not come across it, but there is a whole world of reproduction watches with forums full of enthusiasts and manufacturers providing parts at various price points (believe it or not, there are low-, medium-, and high-quality reproductions), and the investment is not nearly as much as you think it is. The manufacturers of reproduction/aftermarket parts don't have the interest in or motivation to copy every fine detail that a collector cares about, and they are more than happy to use collectors' research to improve their products.
The SM120 / SM300 kits are just the incidental crossover of the two worlds. Other times, people who have honed their craft in the repro market deliberately dip their toe into selling fake watches as legit.