I s what your talking about, i just looked at some of my refractors and the one in my form-400 has a 3/16 rib running around the center of where the refractor meets the gasket, the later refractor doesn't have it, it's just a flat surface. Is your light a form-400 clamshell or a M-400 cobrahead ?
Sorry about the late reply. It is a clam shell. Seems the glass lip is too thick to fit into the slot holders on the metal band with the latch assembly.
Steve