The trade in deal was more of a $50 credit towards a new tank. There were 3 separate exemptions for AL 80's all of which eventually became standardized under the DOT 3AL category. All Luxfer aluminum tanks prior to about mid 1988 used 36351 alloy whether they were done under the earlier exemption or were originally DOT 3AL stamped. Conversely no Catalina tanks were ever made from 6351 alloy regardless of the stamp.
