It is common to find low levels of elements in alloys that are not in the specification. These elements are known as "tramp" elements. Copper is intentionally added to 304 stainless steel at a level of 3%-4% to produce type 304Cu (UNS 30430) stainless steel. The copper in 304Cu reduces the cold work hardening rate and 304Cu is often used as a material for fasteners. I suspect copper at a level of 1% or less in 304 stainless steel will have little effect on its suitability for cryogenic applications. You may want to consult a metallurgist or a stainless steel mill to confirm this is correct and get further information.