I found this old bottle of metal prep fluid in my late father-in-law's garage:
It looks like the Neilson Chemical Company in Windsor was a subsidiary of the American firm, with locations in Detroit and Los Angeles. Google turns up no hits for this company, so presumably it's long gone. So are the "bakery and milk delivery wagons" the product was advertised for.
Interesting look back at how dangerous consumer products were distributed back then. Note that there is no information on the actual chemical constituents of the product, although I suspect phosphoric acid, since that's what's used in current preparations under the "Metalprep" name. This acid is both corrosive and combustible, not exactly "Safe" as the label assures. Mind you, the product claims to be good for, among other things, "Terneplate." This was steel coated with an alloy of tin and lead, sometimes to the tune of 80 percent lead. How bad can that be? "Pre-War Formulation!" Would that be good or bad?