I just got a Hamilton Practice flute in alloy from The Irish Flute Store.
I like the sound and the playability. But I have a question for owners of the same flute:mine came with a quite large “indentation” around the embochure hole, on the opposite site of where you place the lip. It is perfectly made, like if a piece of the nylon/plastic was cut away with a blade.
I wonder if this is normal and if it is, which is the purpose of this ?
Thank you for your answers !
It’s not uncommon for makers to cut away material on the far side of the blowing edge. I’ve seen examples on flutes by Casey Burns and Skip Healy, and on modern flutes (German reformed lip plate). I’ll let more knowledgeable people say what the advantages are.
My understanding is it’s meant to increase volume.
I have that flute.
Compared to other flutes, I found it very easy to get sound from, but hard to express anything. the distance felt like I was blowing into a whistle or something.
But I don’t want to be too derisive of the feature, as there are great flutemakers that use it with “real” flutes, too. 
If you look closely at the lip plate of a concert silver flute, you will see that the curve of the lip plate is parabolic rather than circular, allowing the curve to become steeper immediately past the blowing edge of the embouchure hole. You can do this on a concert flute because the thin lip plate is raised from the thin-bodied headjoint on a riser. This effectively accomplishes the same thing as removing material in the same postion on a circular headjoint. Although I am merely guessing here, I think the purpose is to cause less drag as the air passes over the top of the embouchure, making for an easier-blowing flute.