Anyone know what this thing in the back of the bike is?

[Beekove]

This little box was in the back of the back, numbers are 1-6 with “on” at the top left and “dip” top right. 6 little tiny switches.

 

[Dustin02]

That's the old style speedo healer.

This is the website: https://www.healtech-electronics.com/products/sh/

They might still have the directions posted somewhere if you want to use it.