68 barracuda lower windshield trim??

It's vinyl so the sun can eat it up over time. Most of these cars are well kept today so a good reproduction piece, even without UV protection incorporated, should last a very long time.
As for its function.. It serves as a lock insert for the rubber gasket preventing water leaks. If you did replace it with stainless reveal molding from a hard top for the aesthetics you should put something in the gasket but there again, they don't normally sit out in the weather today.