I think you are looking at the accelerator pump diaphragm and not the vacuum secondary diaphragm. My speed shop needed the list number to get me the correct vacuum secondary diaphragm. For example, my carb is a 4160 series, but it has a list number of 1850.
The 50CC acc pump diaphragm appears to be a special performance part. The next column over in the latest Summit shows an entire kit to install the 50CC version ans says it may require a 1/4" carb spacer. That leads me to believe the 30CC is what most people are going to need. It's $9 cheaper, too!
Get your list number and give Summit or Jegs a call. They should be able to help you. First you need to figure out which diaphragm you need, though. FYI, the vacuum secondary diaphragm does not come in the Holley Trick Kit.