What's the name of the unadjusted one behind the hat?

There is no adjustment at the back of the hat, which is called a closed (fully closed) flat-forehead hat. This kind of hat has different head circumference sizes, so you can choose the one that suits your head circumference. A fully enclosed hat will be more comfortable and beautiful without an adjustment buckle behind it. The hat with adjustable size only needs one production line, which is to reduce the production cost; So instead, a fully sealed hat will be more expensive.