I checked the plywood edges for screw holes this morning where the trim would be mounted. No holes so I don't think there was ever any trim from the start.
Sorry, but I'm still convinced your car should have the cargo area carpeted. Looking at the carpeted seat back makes me lean that way. It makes no sense to me to have the seatback carpeted & not the rest of the cargo area. Previous owners may have replaced that plywood, cleaned the cargo area, and never finished the job. I hope you find out for sure.