i finished restoring my blue olive supra and was noticing the black felts about 3/8 x 1 1/2 inch inside the lugs as they were almost gone from rot i found some material very similar AND PUT THEM IN BEFORE FINAL ASSEMBLY THESE were internal not as a gasket like later models, this made a difference on the sound it was crisper not so ringing as usual on theses ludaloy shells so if there is any input on the factory felts please let me know in theory this acts as a sound deader must of been added from factory after filed trials from the pros in theory .... when was the first ones added keystone era or blue/olive?