window mullion clip
A quick learner from Naples, FL says:
November 22, 2014
I have no brand names or part numbers--really, no information. the clips are only ⅛ inch deep and the mullions snap into them--unless the sun and your window washers destroy the clip prongs, which are meant to slide under the frame around the window glass.
Paul from SWISCO responded:
November 25, 2014
Hello! See our 55-182 grid clip. This appears similar, but make sure our dimensions match yours.