Typically they use some sort of off-the-shelf item- I doubt this is something they made for this brand of board only. At that size, I would expect there was an automatic vent valve involved, part of the plug. This thing was probably injection molded, they would have made hundreds or thousands of the things, that’s how injection molding works.
What I’d do is measure very accurately with calipers and a thread gauge, get the dimensions, maybe do a rough sketch and then have a look around. Amazon, to start with, where a search for ‘surfboard vent plug’ turned up a bunch of possibilities. An interweb search with the actual dimensions would give you far better results. Rather than overcomplicating the issue or reinventing the wheel.
I’d expect the original/maker’s dimensions to be metric rather than SAE/Imperial/inches, choose your measuring gear accordingly.