I've seen CP4's come in two boxes, a silver and black. The silver box stated "CM300's", the Black stated "CM300's and CM100's"
CP4's do not use the attenuator barrel, but, CP1 and CP2 do. The attenuator barrels are different shapes on the two models, not sure about the electronics.
When I purchased my CM100's many moons ago, I was told the CM100-CP4 combo was identical to the CM300-CP4. But, that there were differences when using the other capsules with the two bodies. I assume he knew what he spoke of.