How do I tell the difference between a Motorcraft and Holley Distributor?

I've got a F100 with a straight six in it. The owner wants me to install a petronix unit.....No problem. Called the parts store. There are 2 different model distributors Motorcraft and holley. How the heck do I tell the difference? I'm not having much luck with google....

 

I though Holley only made cool carbs...???
Order one for the Motorcraft distributor and install it...

 

Do you have a part number? Carb Tag??
Year of truck is important also.


.

 

I could be wrong, but in my experience the Holley distributors were usually a governor unit to be used with a diaphragm controlled Holley governed carb. Usually a big truck item. Pickups were usually the Motorcraft.

 

Wasn't the Load-O-Matic a Holley dizzy? What year is the truck?