A number form is the classic example of spatial sequence synesthesia. Ask a number-form synesthete where 37 is and they can point: the number line is not a metaphor for them but a layout with turns, slopes, and distances that never change.
Galton’s 1880 papers in Nature collected dozens of these maps and noted the properties research still confirms: they appear in childhood, they are idiosyncratic, and their owners are often surprised anyone lacks one.
Number forms matter historically because they were evidence, decades before brain imaging, that perception could bind arbitrary content to space in a consistent, involuntary way.