In the 1960s, the contamination of steel was considerable enough that it could no longer be used in medical and scientific applications where precise radioactivity measurements would be performed since the radiation emitted from the steel would hinder the results.
Decontaminated steel could be produced in small batches, but was quite expensive compared to simply sourcing material that was produced prior to 1945.
This began a global hunt for pre-WWII “low-background” steel. Hundreds of tons of steel were subsequently recovered from sunken battleships and old railcars to be used in hospitals, research labs, and various equipment.
This steel nail is called a date nail. These were used by railroads and utility companies until the 1970’s to identify when wood ties and poles were installed. The number on its head is the year it was used. So a nail with “40” would have been nailed into a new railroad tie in 1940, guaranteeing that it was produced before 1945 and is therefore low-background steel.
Fortunately, since the Partial Nuclear Test Ban Treaty was enacted in 1963, global levels of atmospheric radiation have steadily decreased as the contaminants decay into stable isotopes.
Low-background steel is now only required for ultra-sensitive applications, but it remains a relic of a time before radioactive fallout was allowed to contaminate the global supply of air that all life requires for existence.