As discribed to me:  all aspects appear identical to XM-177 except the stock.  This was a heavey wire "skeleton" that folded along the left side of the weapon.  The weapon used M-16 magazines, and the user claimed that the bolt was identical to and interchangable with an M-16 bolt/carrier.  I have never seen or heard of a varient that had a folding buffer.
This weapon was issued as a "CAR-15" sometime during 1963 or 1964 to a SOG unit.
Any guesses?
woody

The "CAR-15" didn't come out until '66.
 

The gun sounds like a M231 port firing weapon but dates are wrong for the M231 didn't get adopted until 1979.



CD

ETA:  Could be a field modification of a shortly with a M3 SMG stock attached.