Rev 1B, the development version, and Rev 2, the first versions sold, were nearly identical.

Some rev 1B boards do not support single step properly. All boards shipped since 12/3/2017 have a modification done to allow single step to work properly. For those with a rev 1B board who want to see if their board has been modified or not, look on the bottom of the board to see if there is a .001 uf capacitor between pins 7 and 8. If the capacitor is present, then the board supports single step.

KIM-1 Clone Rev 2 User manual