Kube is their supposed to be grommets on the front top corners at the end of the channels holding the upper door window w/strips on the convertible frame.
The type you have was only used on the 56 and early 57 t-birds,and I have never seen it reproduced.The cars were different yet.Where did you see a cast aluminum one.
I bought 4 snaps from Dennis Carpenter I think.They fit the 2nd set of holes but the not the 1st set of holes on the belt line behind the doors.I bought another pair but they don't fit either.
Jack Dorn Oakland,Ca.(if that's his real name and address)ripped me off to the toon of $858.00.He took my money,but didn't deliver the Doug Nash trans.