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.