A map can carry more than one moment
A building appears on a map with an early date in its corner. It is tempting to use that image as proof that the building existed then. With a corrected Sanborn atlas, that inference can be wrong: the printed sheet and the information pasted onto it may belong to different years.
The Library of Congress explains that Sanborn began issuing paste-on correction slips around 1920. Catalogues can show both the original publication date and the latest correction date. Its guide uses a Chicago volume printed in 1912 and revised through May 1950 to explain why a reader must not treat the surviving corrected volume as an untouched 1912 view.
Sources: s1
The map was made for a job
These maps were tools for assessing fire-insurance risks. Their detailed building descriptions make them valuable for urban history, but their original purpose shaped what was recorded. A Library blog discusses their use in studying changes to towns, buildings and neighbourhoods. Detail should encourage careful interpretation, not the assumption of completeness.
For a historical claim, record city, volume, sheet, original date and latest correction information. Inspect the key for that edition and check whether the visible structure sits beneath or on a later slip. An independent dated photograph, directory or permit may then help narrow the building's chronology.
A cautious answer is still useful
If a particular alteration cannot be dated, say so. 'Present in the surviving corrected atlas' is a defensible observation. 'Built in the original publication year' needs more evidence. This distinction preserves the map's value while keeping an uncertain construction date from travelling into later histories as an established fact.