Original text by Cecil Williamson: 'Snails by nature are highly sexual and so the folklore of the south west abounds in tales of the powers conferred upon humans in one form or another by the snail. Certain it is that our local witches prepare clusters of snail shells such as this to be lodged around the house as love and fertility charms.'
One type of Halloween love divination involved shutting a snail inside a dish over night, and then studying the snail's trail in the belief that it would form the initial of your future husband or wife (E. and M.A. Radford, 'Encyclopaedia of Supersitions).
Scarborough Museum has a similar charm - a necklace of snail shells strung on red wool - collected in Jersey in 1912, which was used to cure seizures and croup in children (information supplied by Tabitha Cadbury - see her report 'The Clarke Collection of Charms and Amulets' in the museum library).