AMD series 5000, surface mount

I have a house with AMD series 5000 surface mount type , with the use of the Surface Mount Frame which is Type 1, the neck is 3-1/2” smaller than the Ceiling opening. But where the duct meets the diffuser there is no tape, no mastic, no nothing. Is this correct? The house has foam insulation. I cannot google anything about it. Info needed, thanks in advance.