According to Wikipedia:
It began business in Philadelphia in 1772. The name "National Lead Company" was used since 1891 after a series of mergers. National Lead changed its name to NL Industries in 1971.
The name on your lead bar is written in "serifed" lettering, which when stamped into a metal object "usually" indicates the object was made more than 100 years ago, or even much earlier. Anybody who doesn't already know what "serifed" letters are can look at the words United States on a US dollar bill. From the early-1900s onward, most metal-stampings are written in "plain block" lettering (like the word Liberty on a US penny).