Experimental bayonet. Unknown country. USA?

Experimental bayonet, unknown maker, unknown country. Long plain bright finished US M4 type blade with long false edge. 20 cm/ 8 Inch. Hard wood wrap around grip with 19 fine ribs is fixed to the blade tang by two wooden pins, hardly visible. The beautiful grip is widening towards the pommel. Somewhat oval blued pommel with a "single M4 type catch". Opening for the bayonet lug is on top of the pommel. Catch is working and is attached by a screw on the pommel. Crossguard is fixed to the blade tang by two flush steel rivets. Muzzle ring is positioned flush to the blade tang. MRD 15,1 mm.
Crossguard is marked with a J in a box. Total length 34 cm / 13 7/5 Inch. No scabbard.
Very well designed and well made bayonet. Not a fantasie toy.
We do not know what it is. If you know or have any idea, please let us know.