Fuel Cap Gasket

I have read a couple posts on here concerning the fuel cap gasket and two mentions of realgaskets.com and an oil gasket that fits. Does anyone know the part number?

My header tank leaks a wee bit of fuel if I get over 100 MPH, and it doesn't matter if in level flight or descending. I am pretty sure it is the gasket from the way I see it coming out.