Like Tom said, that looks like a mounting stud for your exhaust or intake manifold, so external. You know the bolt you attach your exhaust pipe to? It is one of those. If you have two of those for your exhaust and two for your intake manifold (the pipe your carburetor attaches to, but on the engine side), then it is probably just an extra.
Here is a picture of the intake studs on my motor...
Looking at that picture with my piston at TDC makes me really wish I had tried harder to get my cylinder off to trim the piston skirt. I tried a little, when it did not come off easily i just figured it was best to not force it until I had to take it off at some point in the future. Oh well, who needs extra power?