That black o ring was probably for your bubble counter. It seal's the top plate on the bubble counter. As the other guys have mentioned, mine too had a plastic/teflon washer that goes in between the tank and regulator.
When I got my regulator I had 2 of them ziptied to the regulator unit.
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No matter what the morrow brings, inventors keep inventing things.
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Jonathan
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www.cakerybakery.ca
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