This button is made from caste magnetic white metal (?steel). Is it a fashion button or a uniform button? Boomerangs do appear in some military buttons (e.g. Duntroon Miltary College).
Semco Silkette was advertised for sale from1932 until 1954 or later. From the ad below it appears that during the war it was unavailable (silk being required for things like parachutes). Times were obviously still tough in the U.K. for the shipping of food parcels to be kind gesture.