I detected this Button today. It has an Eagle with the word, Waterloo under it and below that what I think are four letters, R N E D. On the back of the button I can make out the word, London and than the letter , J. with the rest of the writing unreadable.
Yes part of a buckle like the one you link, but the linked one is a much earlier version. Strap fitting is just that a fitting on a strap/belt, ie. a buckle or sword hook or strapend.