An imperial unit (let's remember we got this from the Brits who now say they're metric... but are they?) is generally based on something in day-to-day life so they're relevant. They would have probably been named in the late 40's or early 50's. So I suspect the they'd be based on ways data was transmitted then.
4 taps (like on a telegraph) = 1 character
so 1 tap is 2 bits
1 sheet (like paper) = 13,000 characters
so 1 sheet = 52,000 taps = 104,000 bits
... etc
1 bankbox = 500 sheets = 26 million taps = 52 million bits
It's 1 to 4 for the English alphabet, though only E is a 1 tap. I started with 3 taps = 1 character but then all the whole number in my examples go away.