For your consideration, here is a Vintage Fluid Drive Chrome Cap, part number LC3P720.  It has a diameter of 3 1/8 inches.  It has some wear and pitting on it.  Hard to get a good photo of it.  It will be mailed in a box for protection.  Item pictured is the actual item up for sale and comes from a non-smoking household.  (409.911y-.7bx3)