What does a .1 suffix on a PG data file signify?

I'm investigating a 'could not stat file' error that points to a file "base/16384/52212305.1".  All the data files I've ever seen have names that are whole numbers; I've never seen a decimal suffix.  It occurs to me that perhaps this is some kind of temp-file, or a system for avoiding duplicate file names, both of which might be relevant in the context of this error.