The heraldic "lozenge" shield is a diamond. But yes.

As for cadency marks, we all agree they exist, but even the College of Arms is of the opinion that piling them on top of one another makes for an absurd result, and the whole coat passes to the heirs of an armiger, who may use cadency marks if they wish.

Wikipedia also mentions several letters from the Garter Principal King of Arms to the same effect, and the 1911 Encyclopedia Brittanica. Several of my heraldry books say the same thing.

I think we're all right here. The marks exist. Sometimes, they're used. I'm just saying they're far from universal and are not required outside of Scotland.