Booked 2 Hiltons today... The Boca Raton Resort (Waldorf) and an Embassy Suites in Seattle (opposite sides of the country AND different brand categories)... Both of the emails said Tier Status B (the ALT text in place of the header image).

First time I noticed it, so I went back and looked at my other Hilton confirmations and note that the first time I saw this was when I made a reservation on September 7th. My last booking before then was placed on Aug 21 and that email confirmation did not have this part (though it was a European reservation).

The B is definitely just some back-end code or placeholder for Diamond, judging by how the text is replaced by an image.