If I had to guess, I'd say that you're referring to StuffIt archives (with a .sit file extension). They were the most common classic MacOS archive format. Copies of StuffItExpander (free, if I remember correctly) are readily available, however unrar on *nix platforms will also extract them and retain the Mac resource fork according to https://github.com/PiSCSI/piscsi/issues/630 . Good luck!