Of course, this also means that you can't use the wrench on SAE or metric or, for that matter, Mesopotamian fasteners 

B.S.F. likely refers to "British Standard Fine" nuts/boltheads.  As I understand it, the British used differently sized nuts/boltheads for fine thread (BSF) than for standard thread (British Standard Whitworth, BSW).  You'll often find "Whitworth" wrenches marked "[X size] BSW/[Y size] BSF," referring to the major diameter of the thread, NOT to the nut dimension.  So the 1/2" BSF max on the range would mean it fits the nut for a 1/2" BSF bolt, although it's pretty short for removing a well-torqued/well-rusted nut that size.

Imagine being a British car/motorcycle mechanic, machinery mechanic, millwright, etc., and having to pick the right wrench for a nut you encountered on a repair.  You'd probably get pretty good at it, just as mechanics/repairfolk working on equipment with inch/fractional and metric parts: look at it, pick the right wrench.  But it might take longer.

I would like to learn as much as I can about a large square I picked up. Overall dimensions are approximately 30" X 12". I can't find a brand name on it. The blade part is stamped "Cast Steel" and "Warranted". There are no graduations. The black part appears to have been painted by hand. Would this be used in a particular industry such as shipbuilding? How common are squares this size? How old might it be? Would this be of interest to a collector?

 Congrats Jim! 

I think it is tied to one of the forerunner brands to Proto. Check on Alloy Artifacts for the last word on it. Likely 1940's vintage IMHO.