The letter in the original post is dated June 1940, over a year before the first experimental gassing at Auschwitz in September 1941 whose victims consisted of Soviet PoWs and a group of Polish inmates so is not related to subsequent murderous use of Zyklon B in the death camps.
I agree with others this can opener is an old can opener and may have some "collectible" merit in that fact, rather than being associated with anything more sinister.
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