Do clamshell fixtures with FCO optics exist? I know that there were weird optics on certain luminaires, such as SCO high masts, FCO NEMA heads, etc.

From the manufacturer , I'd say no. It is possible however that one or more could have been converted to FCO in the field by it's owner .

Didn't the Form 400 use the same shape glass as the later M-400 series luminaires? If so, maybe you can stick a glass from a FCO M-400R2 into a Form 400.

Anyway I believe the first FCO luminaire was either the GE M-100 or the Westinghouse MO-8. Both of which were low cost open bottom luminaires for residential streets.

 I  do know there sky gaurd made that u can put on nema heads.

I guess you could make it FCO by taking off the refractor.

A lot of lights used for billboards resemble clamshells with flat lenses.