The back of the clasp is stamped Nippy-Clip Made In England Imitation.
Nippy-Clip was a product name of Stratton Of Mayfair Ltd in the 1950s and 1960s.
Imitation is the UK term for fashion jewellery, i.e. not precious metal.
Stratton was established in the UK in 1860 and is a market leader in its field. Stratton’s world famous name is, and always has been, synonymous with both quality and design.