Imperial C and D poles are almost identical to each other. If you are looking for a replacement post for a current system, you can measure the diameter of the post. If the diameter of the pole is 2 3/8″, you have a C pole!
Imperial D poles are a bit larger than the C. You can measure the diameter of your current pole to get an appropriate replacement part. If your pole is measuring 3″ in diameter, your system has a D pole!