Joseph Priestley Medal 1804 by Thomas Halliday

I have been looking a little closer at a Joseph Priestley medallion that I purchased some time ago. I had initially thought that it was the 1804 Death Medal by Thomas Halliday, BHM 563 shown above. The attribution for this medal is a size of 51mm and a composition of white metal. BHM 563 is inscribed with the name HALLIDAY on the obverse side on the lower edge of the bas relief bust. Now, after careful inspection my medal is quite different. It is 52mm diameter and is apparently made of bronze.

Furthermore, the actual portrait of Priestley is slightly different, primarily in the shoulder area and other parts of the bust below the face. The inscription reads "HALLIDAY F". There is no question that the medal I have is not the BHM 563 Halliday medal. I have posted below a picture of my medal.

I have now found my medal listed on the British National Portrait Gallery. It is listed as NPG 175b. According to the NPG this medal is also by Thomas Halliday and also referred to as the Halliday 1804 Death Medal and is 51mm in diameter and is made of bronze and has the same shoulder detail as my medal with the HALLIDAY F inscription.