Don, my late '73/early '74 3c seat had a steel base (thin, got very rusty) but didn't have the steel inserts attached like in the pictures, just had slits in the seat base panel (like the fibreglass one pictured). There might have been some extra stiffening inside the slits, I don't remember if I ever looked, but that was definitely an early factory seat. And it did look just like the one in your picture (drool).
I think the metal inserts you're looking for are some sort of aftermarket thing, I've never seen them on a seat before. I'd keep looking for another 3c seat, they're not that rare, plus there are plastic replicas (that don't rust). If it's not a replica and you're not concerned with total authenticity, that's what I'd use.
Ken