yes, SIT! is a creator type for StuffIt Deluxe (Expander is SITx). SIT! is also the file type for StuffIt 1.5 files. StuffIt 2.0-4.0 uses 'SITD' filetype and Stuffit 5.0 filetype is 'SIT5'.
My script checks the internals and sets the filetype metadata accordingly.