Question on fan pulley

[Afordman31]

When replacing the fan pulley on the crankshaft there is a keyway inside the pulley and a notch. I first slid the pulley on using the keyway, tightened the bolt, rolled over the engine by hand and it seemed to hang up. Loosened the pulley and installed it using the notch in the pulley then could roll it over by hand with out it hanging up. Why does the pulley have a keyway and a notch?
This is confusing on which to use. This is on a 37 flathead in a 40 ford.