Since the likely candidate for the XP-904 slot was the XP-23 fighter, which was NOT a monoplane, I wouldn't discard MX-904 straightaway.
Here is what that designation covered:
Generalized supersonic bomber-launched air-to-air missile study (final result was XAAM-A-2 / XF-98 / GAR-1 Falcon); see also MX-1005-D
So this was a supersonic air-to-air missile. At the time of that competition, Curtiss-Wright was still in the hardware manufacturing business, and even had a couple of missile and target programs on their hands (XSSM-A-23 Dart, Hypersonic Test Vehicle), so it's not far-fetched to consider they might have submitted a design proposal to MX-904.
I'll agree that the term "monoplane" is awkward for that kind of vehicle, though, but who knows?