Narishige's SM-15M adds Y-axis movement to the SM-15. The standard SM-15 has X- and Z-axis movement units, but no Y-axis movement (i.e. the same direction as the AP frame bar). This one-axis fine movement unit can therefore be placed between the AP frame bar attachment point and the SM-15 body. This enables smoother, more precise adjustment than was possible by the former method of adjusting the Y-axis with the sliding AP frame bar.