OOF2: The Manual
You're right. It should be “OOF2.1”. But it's not. OOF2 is sufficiently different from OOF, that calling it “OOF 2” seemed insufficient, so it was called “OOF2”. But calling the first version of OOF2 “OOF2 1.0” didn't seem right, so we called it “OOF2 2.0”, and are calling the new version “OOF2 2.1”. We might call the next version “OOF3D 3.0” if we feel like it.
We're not going to release alpha or beta versions. All versions of OOF2 are tested before they're released, but we can't promise that they are bug free. When we fix bugs, we'll release a new version. When we release a new version, we'll increment the version number.