Ours so far have no casque, but I will keep an eye on them as they mature to full out adult* which should be soon* and see if they grow one.
What if they don't grow a casque like in the pic, but still have blue heads?
They could be like someone else suggested Helmeted Guineas. They do not have the bone thing on their heads, yet, but it does look like some of them may be forming one, and I'll watch that to see what happens. Though they were sold to my dad as pearl.