My TC cam cover was black with aluminum ribs. It was my understanding that all the big valve motors all had this cam cover (see attached pictures) but I might have heard of a big valve motor with the un-ribbed cam cover that I think you're referring to. Since the ribbed cam cover had "BIG VALVE" cast into it, they should have only originally been installed on big valve motors but then with Lotus, who knows what might have happened!
As I was searching for a good picture to post, I was astounded at how long it took for me to find one like the ones I was used to (at least as found on all the Europa TCs I've seen in the US)! There are clearly many different versions of them and owners often paint them (or not) to their taste.