The biggest thing is ACT/SAT approval. If that’s what you need, check out NumWorks[0]. It works on both and is completely open source (down to the schematics).
It's source-available, not open-source. And the schematics sources are not available; but I don't think it would be very difficult to reinvent them, considering the number of parts is small.
On a lighter note, they make it very easy to hack on the calculator, it's written in C++ and in a Git repo at Github.
Granted, it’s not the entire device, but it’s more than most products will give you (none). You are correct about it actually being source-available. My mistake there.
[0]: https://www.numworks.com/