The reason is that this thermos cup is not made of 304 stainless steel.

304 stainless steel is a high-quality stainless steel material with corrosion resistance, high temperature resistance and oxidation resistance. It is widely used in food containers and kitchen utensils, including thermos cups. If a thermos cup does not have the 304 mark, it is likely that it is made of other types of stainless steel or even non-metallic materials.

It should be noted that the country does not have mandatory regulations on the labeling of 304, so some thermos cups may not have the 304 mark, but this does not mean that their quality is substandard or that materials harmful to health are used.