Thanks Old Guy,
Very useful source of caps.So, if i understand you correctly you dont recommend to get rid of the alu cap and replace it by a felt cap but rather to cover the alu cap by felt that should be glued on the internal edge of the medium/treeble of the 420 and not on the aluminium cap itself.I would have thought it had to adhere to the cap to dampen the overtones properly.For the glue,i will try to find a reconer and ask for his advice.i will keep you informed.
Charles.