That's right, it should be upside down. Nobody really understood the reason for it but the reason put forwards was that of target obscuration using the ground glass graticle post that was necessary at the time if you wanted some sort of illumination of the graticle post with a prismatic telescope. If it was the 'right' way up, then you obliterate a large part of the target.

This didn't seem to be of any consequence when the later same meat, different gravy SUSAT was introduced later