I'm by no means an OE aficionado but my conv had that plastic piece Ulf. My hardtop did not, but the one on my hardtop looks like an aftermarket replacement. Hope this doesn't confuse things. At least its easy to slap on if need be.
I stopped by my friend Johan on my way back home from work yesterday and checked out his 65 Dart 20 w/V8. That car was assembled October 15 1964 and had the plastic shield. After talking with Dan (slantsixdan) last night I think that around late September or early October 1964 Chrysler started adding the plastic shield. If they did it on all plants and all assembly lines I'm not sure about at this time.
I can't recall ever seen a aftermarket replacement shield on a starter relay!