From the Wikipedia:

Arsphenamine was marketed under the trade name Salvarsan in 1910. It was also called 606,[1] because it was the 606th compound synthesized for testing [In Germany it was the trend to designate compounds by their development number. Another compound known commonly in Germany by its number is Parathion, which was the 605th compound to be developed in search for insecticide. It is commonly known as E605 (E stands for Entwicklungsnummer (German for "development number")]. Salvarsan was the first organic anti-syphillitic, and a great improvement over the inorganic mercury compounds that had been used previously. A more soluble (but slightly less effective) arsenical compound, Neosalvarsan, (neoarsphenamine), became available in 1912. These arsenical compounds came with considerable risk of side effects, and they were supplanted as treatments for syphilis in the 1940s by penicillin.

Note the reference to inorganic mercury compounds. Pills of white mercury, no doubt. I am still pretty sure they were mercurous chloride (Hg2Cl2), not to be confused with its very deadly cousin, mercuric chloride (HgCl2), also known as bichloride of mercury. Both will kill the spirochete, but the mercuric chloride will kill the host as well. With mercurous chloride, you have a fighting chance of finding a dose that will kill the little corkscrew buggers but not do you in at the same time.