My friend Marc Gettemans from Belgium has come up with research that may well provide the answer. On page 45 of Trade & Hallmarks on Firearms in Belgium there is a note that in 1903 Clement Charles Neumann Freres registered the following mark: two crossed hammers with the inscription "Hammered Fluid Steel" and with the letters "CJCJ" in the angles of the hammers. Although the mark on my F grade Lefever is very worn, the "J" is visible and "HA" may be the beginning of "HAMMERED FLUID STEEL". The Belgian trade & Hallmark book's reference to 1903 or so is a reasonable fit for both my F grade and the Lefever Arms catalogues which reference "Premier Steel". The Belgian producer appears to have meant to use the mark for export. Hence the English version "HAMMERED FLUID STEEL". The company, as noted on the same page of Marc's book, also registered a German language version, "Prima Fluss Stahl", and a French language version, "Acier Special". Many thanks, Marc, for this great info.