Malt syrup is a sweetener made from barley but is also an ingredient in beer making.
As you might expect, breweries either had to adapt or close when Prohibition was enacted. From http://www.digitalhistory.uh.edu/database/article_display.cfm?HHID=441
"Anheiser-Busch made it through Prohibition by making ice cream, near beer, corn syrup, ginger ale, root beer, yeast, malt extract, refrigerated cabinets, and automobile and truck bodies. "
If you make a quick search on ebay for "malt syrup" you will find lots of advertising, labels, etc. for earlier 20th c. malt syrup products.
