Base pairs. I don't know the exact math and haven't ever done Westerns, but there's almost certainly a way of converting protein size (typically expressed in kDa) to base pairs, since theoretically a large enough quantity of bp would weigh as much as a protein of a given size.
The trick here is to understand that Western blots use gel electrophoresis to detect protein size, meaning that heavier proteins will not traverse as far as lighter proteins.
Come on. Having actually done Westerns, there is absolutely zero chance that the "bp"=base pair in that figure is on purpose. Western blot is a protein blot and they are always measured by their molecular weight (kDa). Base pairs would make no sense here. The numbers in the image are obviously kilo daltons.
How do we know the numbers are kilo daltons? Well, they say they're using β-tubulin as a loading control. β-tubulin's molecular weight is about 50kDa and that's what you can see in the image.
Also, look how the headers are not aligned properly in the image. As if the image is a draft.
The trick here is to understand that Western blots use gel electrophoresis to detect protein size, meaning that heavier proteins will not traverse as far as lighter proteins.