based on what I can see
It looks like a 33-36 ford cabriolet header clamp
The slant window cabriolet and A400 are similar
But Briggs also made cabriolets for Chryslers,Desotos,and Plymouths and they use a very similar clamp
I have seen quite a few different one similar to this one
the angle of the flat sides differ,the hole spacing differs and the width differs