John,
Begining to embrace that De Tomaso consistently inconsistent at least. I don't have an emissions tag. (Vehicle was hydro blasted to bare metal, no holes for emissions plate) All others are as described but I also don't have the original valve cover. It may have possibly been mounted there I guess. Just the same would be nice to have one.
A quick search on Mustangs says they were in the top of the valve cover (as I remember also) or on the side.
Just the same I may pick a plate up and mount it below the motor # tag.