I'm getting tired of having to explain thirty-five-year-old tech to newbies...
It's a multivolume zip. You need ALL the parts in your disk. Then you use your trusty zip utility, like 7-zip, Nanazip or such (I don't know if Microsoft's crappy built-in zip support actually supports multivolume files, I never use it) to open the *first* file on the set. It will treat all the set as a single large zip file.