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Alright, lets look at the US Customary Units and their definitions. Here is the section called “Mass and Weight”. As you can see, everything is defined in metric units of mass. You won’t find even pound-force to be part of the Customary units. I couldn’t find any source saying that pound (not “pound-force”) is a unit of force. However, there was an agreement (I think in 1955) to define the pound in kg.
That basically implies that lb (pound, imperial unit) is a unit of mass and “pound-force” (non-imperial unit, part of the British Engineering Units) is a unit of force. Thus, pound (on its own) is a unit of mass, right?
British Engineering Units are not a part of the US-Imperial System, but since I never specified I suppose it’s a good argument.