They had used 26.4 posts previously, but that was when their seat tubes were ED28.6. For the P7 and C16R they went over to ED31.8 seat tubes as a design feature in the interests of greater stiffness there. They could have used a 29.8 or 30.0 post, but they weren't in great supply in those days and anyway were much heavier than a 26.4 (and it is a steel post I seem to recall). Orange were far from alone in using shims when designing a frame with a 31.8 seat tube. e.g., all USE posts were designed to be used with a shim.