FWIW, some photos.
Photo of the shaft diameter difference between a PS and manual box. The calipers were set off of the PS shaft. The minor diameter (diameter closest to the nut) looks to be pretty close on both pitman arms. The difference is in the taper of the shaft. The PS shaft has a much greater taper so the the other side of the spline hole is much larger on the PS '881 arm compared to the MS '881 arm. The calipers are showing this difference.
Photo of the pitman arm markings from a 1970 Z28. Power steering arm on top and manual steering arm on bottom. The manual steering arm is stamped with a letter "M" at the spline area. Arms are identical length with only difference the spine diameter size.
