I've seen that button before but can't remember where I saw it - maybe on here.
That's a great find, congrats!
It will be a famous person from the 18th C, who probably was a Dragoon/Military leader.It looks like a Dragoon motif. Are we looking at a Dragoon Cufflink? Or like Smokey said a livery button and that is a riding crop??
William (D?) Cumberland.I think it says W.D. C U M