That NaPTAN dataset only covers England, Scotland and Wales, not NI (not sure why - probably just Translink being useless).
It’s a while since I’ve visited the Ulster Folk Museum but I don’t remember being able to send letters from the post office. (Similar they can’t arrest you at their police station 
) I reckon a ‘historic:’ tag works fine (it’s clearly on the museum site, which is a big open air one)