What the heck is the "topic" of this post? I'm seeing, in various places, "man page", "main page", and "info pages"! The "info" page is generated from the same "source code" as other formats of the Manual, and should match the actual version (in an ideal world...). The "man page", in contrast, is updated separately, if at all. It doesn't mention a "version" specifically, but still claims that "-O0" is the default.
As Mahmoud says, "nasm -v" should indicate the actual version (and when it was built) as reliably as anything...
Best,
Frank