Keith, can you attached an image of the top side of both knobs?
Do you remember where you bought your new knob?

I had no problem using the knob from Classic Parts and from Jim Carter (and, Chevs of the 40s looks correct).
Attachments
wiper knob chevs of 40s.jpg (41.49 KB, 132 downloads)
wipeer knob classic parts chevy duty.jpg (25.42 KB, 131 downloads)