They are effectively the same. It is a bit of a holdout from the early grading system. In the hammer gun days there was a myriad of grade letters for the guns. The gauge and also grip configuration determined the grade. Letters like R, T and U were used for grades, among many others.
The NH was a hammerless 10 or 8g and it came later when the N was dropped as a separate grade that they all just became PH grades. Regardless of gauge.
A study of the grades section of TPS would be worthwhile. As well as period catalogs.
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B. Dudley
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