I have six FAD nickel buckles one of which is an anodized specimen. All of my nickel ones are what you would expect to see in a nickel silver buckle. I suspect that yours has changed color due to some type of exposure to heat or even being in the ground. I do not believe these buckles were ever treated by the producers to be golden in color. Whatever, it is still a nice buckle.
By the way by any chance are you on Skype?
A little tongue in cheek when I made the "golden" reference.
The buckle is die stamped BRASS with originally, a very light nickle silver plating which has all but disappeared. It is as simple as that.
What caught my eye though was the brass construction, the ear less box and the early pin and claw arrangement. A first for myself at least.
I will and when time permits make a comparison of roundel designs to other FAD buckles that I hold and hopefully confirm and as I suspect, that this buckle was made by Assmann.
I do use Skype Bill and I will send you my "user name" by private email.