Its not so much the B and C types, they are generally referred to as 'Internal' and 'External'.
The earlier external type split into three just above the chest area. One cable went to each earphone, and the third up to a microphone. In that way the two were akin to a stethascope and it meant that the microphone on the loom was permanently attached to the helmet.
The internal type basically remained as one cable which attached to the back of the helmet. Again it split into three but each cable was now sandwiched between the inner and outer layers of the helmet (IE it was now internal), with one cable still going to the earphones but the third to a plug which was affixed to the left cheek area of the helmet. The arrangement was much tidier and easier to use, and it meant that the microphone (and oxy mask) could be removed and changed, should it become damaged or superceded by a newer model.
The external loom is generally found on the B Type helmets, and early 'external' versions of the C, D and E Type helmets, while the internal loom is later, and found on later versions (internal) of the C, D and E Type helmets.