I have been assembling clubs for myself for several years now (20+)...doing regrip work and reshaft work for friends..... Today I was switching out a club shaft on my "Calloway ST Max D" 3 wood and ran into something I've not seen before. After removing the head from the stock shaft....I noticed a clear-ish rubber-feeling plug protruding from the body of the head to the top of the hosel. Does anyone know exactly what this might be. Didn't really cause problems as the new shaft fit around it and seated just fine. Just wondered what it is ... and what it's purpose is?