This is a rare piece indeed!  It is a Colt Commercial BAR Buttstock (NOT A COLT MILITARY M1918).  It has the C Arrow mark on the heel and has a FN Monopod adapter added to it.  It also IS NOT inletted for the trapdoor butt plate; a dead giveaway for Military stocks. 

Fair condition, with dents and dings, the stock is cracked along the buffer tube on the left side (fixable),  I think this is either a FN Model 22 or 24 buttstock.  Comes with monopod mount bracket but no other hardware.