The mystery of the Royal Marine crest

Stuck in doors today so a home type blip. 

Many years ago my wife's sister moved into a cottage near Exeter and fixed to a beam in one of the rooms was this crest, it is made out of solid brass and measured 5inch by 4inch by 1.5inch in width  and weighed 2lb in weight.
When they moved from the cottage they passed it to me and even though I've tried to locate an owner,  of what is a substantial item, it still sits here on my desk.