I've only inspected 3 HP pieces but all are right at 10.5." Most of them will have testing marks on the face that are like 1/16" round dots as seen in the below photo. This particular one has a dot in three places. Measuring the distance between the pair of mounting holes on each of the three legs of the steel hat should be right at about 3 1/8" from center to center. Also, the early HP pieces will have all light green springs.