thanx for the reply.
here is some explanation:
when we design a flyback transformer, we cant get directly our required Primary inductance(Lp), without keeping Airgap.
in india, Normally vendors use some milar sheets to get air gap b/w cores.
but when we get them china, they use other technique like they will reduce the dimension of one core such that the air gap is achieved.
Like wise , they will reduce the dimension of 2 cores such that the air gap is achieved .