Aramis Antiques has dated this piece to the 1930s.
The initials match those of Danish jewellery designer Hugo Grun and Hugo Grun & Co, who together were active between 1915 & 1985. The brooch is designed in the manner of early 20thc renowned silversmith David Andersen. The silver and enamel are in perfect condition, no damage.
Aramis Antiques has dated this piece to the 1930s.
You can view the stock and full dealer information on the Aramis Antiques Profile Page