This is from the camera wiki: "The Spectra range of cameras also sport better lenses on average than the 600-film range, with most of the models utilising an arc-shaped range of focusing lenses inside the body that swing across the exterior lens element to provide correct focusing, rather than adjusting the distance between internal lens elements."
They called the lens "Quintic" it is a 3 element lens. I think this is a case that using plastic allowed a solution that may not have been possible or feasible with glass optics.