Yes, the battery tube is just a little bit bigger (~42mm vs ~39mm on the D lights) so they can fit 4AA across without boring. The body tube on the C size is different also, not sure by how much since I don't have one I can measure. But I once got shipped old C switches by mistake and they were to big to go in my new C light. The switch is a little different on the D also. It has a seperate base and bulb post just like the current C sized switches. And it is also held in with a screw in retaining ring insead of the fixed snap ring on the current ones. There is also a difference in the heads. The old ones did not have the threads inside the head go all the way through, there was a smaller diameter lip at the top of the threads. The newer ones have the threads run all the way through at a constant diameter. However, this particular change did not occur at the same time as the serial number/body diameter change but must have been done later. There are some smaller diameter lights with the D serial number that still have the lip in the heads.
