Looking at Benelli's 2006 catalog, the straight stock is an option for the M4. The stock, both straight and pistol grip, appear identical to the older M1/M3 stocks but I'm not entirely sure they are in fact so. The catalog shows in the parts section that the M1 nad M3 stocks are the same as the new M2s but looking at guns, the rear of the trigger guard are slightly different between the M1/M3 and M2. Maybe the part of the pistol grip stock that mates the receiver at that point is soft enought to comform to the different shapes. (Not a problem with the straight stocks.)