Also true, the pre-blackened mesh was added after painting. The production line had painted and completed vents lined up ready to go to suit car being assembled, of which most was also already painted.
Here's photos of a NOS fender vent sold as a replacement spare part, albeit in light grey primer instead of finished paint including inside the vent housing. In this scenario they'd need to mask up the mesh, or respray, or simply ignore was fairly common too. Also note that the rear doesn't have any primer or overspray so that the mesh goo has a good fix onto the fender material. It also shows that the original mounting studs were also glued into place with a resin looking epoxy, not a tight threaded fit that can damage and distort over time.
There's a possibility that the vents could also have used an insert template for shops and dealers doing touch-ups and resprays a lot. Formula Hood vents had these on replacements as did something else that's escaped my memory at present.
NOS GM 70-81 Trans Am RH side fender extractor vent 480796 (2) by
Ben, on Flickr
NOS GM 70-81 Trans Am RH side fender extractor vent 480796 (3) by
Ben, on Flickr