Recently listed on eBay, 2 pressed glass salts – one similar to yours, Keith, and the other also with a swan motif to the base but with the addition of a British RD number 400564 (registered by M & J Guggenheim on 11 November, 1902. Both are about 7.5cm long.
The Guggenheims were importers of chinaware, and of glassware. They registered 16 designs between 1897 to 1938, but were not manufacturers, and it is thought that many of their registered items were actually made by the Libochovice Glass Company in Czechoslovakia.
Open salts are notoriously difficult to attribute, so it’s quite gratifying to find an example with a registration number.
(Permission for the re-use of these images on GMB granted by mr_minou).