RE:Gurus, please advice. Is the new ...
Whatever date you see under "Expires on" section, only that much the visa is valid. Usually officers allow until the date, your 797 was approved. It is upto an officer to decide until what date the visa should be valid(less than 797 date).
It happened to some people that they got approved for the dates less than the 797 indicates.
I am not sure(or heard of) the visa issuance is related to the country of the person.
To my knowledge, St Louis works exactly how the officers in the US consulates work.
You may want to talk to your immigration attorney.