Hi Alan thank you for writing in. I can't see any reason to doubt this pair of cufflinks are gold. And you say they are marked with 18 ct gold standard mark.
I can't comment on the letters you are not sure about. Also lozenge shape is usually equated with a diamond shape.
http://en.wikipedia.org/wiki/Lozenge
If the letters FA are in a octahedron this mark was used by Francis Adam (mostly Boxes & Accessories, London, UK) - ca 1930s - 1960s
Perhaps made under import.
Any case, you did very well! A handsome pair of art deco period cufflinks indeed.
