the Form 400 here is also the rarer later generation one....you can distinguish it because its boxier near the slipfitter,has a seprate reflector and has ribs in the side of the fixture body.
They are actually a little bit different, not exactly the same....but yes it will work, if you BEND the back rim of the clamshell door since the thickness is a bit different...it would have to be forced bend....to fit..