I'd call the vendor you bought it from and explain the situation. They should be glad to ship you the correct knob you need.
What I don't like is when you order a part that requires fastners and for instance say a knob like that, they sell the fastener seperatly from the knob but don't explain that in their catelog. Seems like they have to nickle and dime you to death on some stuff.
Note to Carpenter, Drake, Vintique or whomever......if the reproduction part you sell requires fasteners include them with the part, not sold seperately. It's just common sense and courtesy.