The roofline hasn't changed since 2016; it was refreshed for 2019, when AWD-e was added as an option--and those cars have a different muffler and lack some underbody paneling at the rear. Perhaps you're thinking of the different rear glass and spoiler on the Prius Prime?
(2016 Prius)
(2019 Prius)
(2019 Prius Prime)
matthewmillerwriter, do you have pics of the fins? I'm curious what you're seeing.