I can't answer your question. The four authors of The Parker Story, tabulated the guns, in each grade section, as "Lifter-Action" (in one case, just 'Lifter') "Top-Action", and "Hammerless".
Price, The only one of the four original Authors, co-authored the serialization book with Fjestad, and only in that publication is the "U" used to describe the type of action.
So, Who made the decision to use the letter "U" ? Why not "L" ? Certainly not because 'L' could be confused with 'Laminated', because they aren't the same table column, just as "U" is in the Action column, but there IS a documented use of the letter in the grades of gun.
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