Full Idea
Conventionalism is apparently inapplicable to Kripke's and Putnam's examples of the necessary a posteriori (and, relatedly, to de re modality).
Gist of Idea
Conventionalism doesn't seem to apply to examples of the necessary a posteriori
Source
Theodore Sider (Writing the Book of the World [2011], 12.1)
Book Reference
Sider,Theodore: 'Writing the Book of the World' [OUP 2011], p.268
A Reaction
[Sidelle 1989 discusses this]