The process of comparing industry over time is complicated by the fact that the Census Bureau’s classification scheme changed many times since it began asking about industry in 1910. To combat this comparability problem, IPUMS-USA created IND1950 to aid those interested in long-term analysis. If you are comparing the industries in 1880 and 1940, keep in mind that in 1880, industry was not collected directly, so the IND1950 values for the 1880 census are inferred from the respondents’ reported occupation.
To understand how the original 1880 and 1940 industry classification were transformed into IND1950, I am going to point you in the direction of a few resources. First, you can find detailed information about how IND1950 was created at this page. Second, the original 1940 industry codes (IND) can be found here, and the original 1880 OCC codes can be found here.
The translation table that IPUMS uses to generate the IND1950 variable is available on the IPUMS-USA website and can act as a crosswalk. You can read about translation tables hereto get familiar with the layout and then download the IND1950 translation table here.
I hope this helps.