Save selection defaults to 4/4 time signature, but no display
Create a score with a time signature besides 4/4.
Select a section of the score that does not have a time signature in the first selected measure
Use File->Save Selection...
Result, the file starts in 4/4 time, but does not display any time signature
If the first selected measure has a time signature, it is included in the file. Time signature changes within the selection are also honored.
I made this major, because rhythms are likely to be rewritten when the proper time signature is applied and it will take time to fix them.