Topic: 1970 mustang mirror gasket

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Anybody know when or why the RH racing dual mirror gasket had a change ?   I think it was a late 70 change because I never seen this gasket change on any fall 69 built bosses I worked on but I recently found a second  rh dual mirror gasket with FOMOCO DOZB-17724-A & the service replacement gaskets had FORD so I know these 2 DOZB gaskets are used assembly line type but  they are thicker than the C9ZB & I only found the RH side so am wondering if this is a plant or vendor change & if only rh side & a very late 70 mustang change?