Trace,
It should be a stud between the valves; sounds like yours has been replaced with a bolt? The first nut on the stud secures the cylinder to the crankcase. The second clamps the tappet cover eye bolt to the stud. You can see the two nuts straddling the eye bolt in the picture Roy mentioned.
Do not back out the stud to make it longer, it needs all the purchase into the crankcase threads that it can get! You may need to find and fit longer studs to suit. I am pretty sure the same stud was used right through to the 1936 250/350cc models.
-Doug