Here's the image, dated 1937, which says "You must keep up treatments for at least six months to one year following infection."
The key drugs for treating syphilis at this point, before the development of modern antibiotics, were Salversan and its derivatives, arsenic compounds first used in the 1910s. It simply wasn't as effective an antibiotic as those developed since that time. As you might expect, drugs with arsenic could also have some serious side effects.
