Just to try and clarify the 'correct' RD numbers for the various Davidson patterns:
Dave's salt, in the first post, is correctly marked 549518, registered 18 September 1909.
John's salt, in the second post, is an example of Davidson's registered design number 413701, registered on 14 July 1903 (their 1904 suite - a.k.a. 'William and Mary', mainly by American collectors). So the 'correct' plunger for it should also be marked 413701.
I attach two photos of pink RD 413701 salts (one circular, one oval) both correctly marked 413701.