Since no one else has responded, I'll offer my assessment. I, too, experienced the phenomena you described and isolated FMStrs.dls as containing information specific to the runtime files. FMStrs.dls is a database of all the text strings for runtime messages, in the selected language. But, obviously, there's more to it than that.
Regarding updates to a runtime, I replace only the built runtime files and FMStrs.dls. I've been doing so for over two years with no known problems. I wish, of course, that someone with knowledge of the internals would chime in with a true explanation of the file's contents and function.
I hope this helps.
I have a feeling this only becomes necessary when the developer changes which file is the primary file for the solution, or perhaps renames it.