My hunch is that the PRC is SHORT by a few cm, not long.
In my Optickle simulation, the sidebands are not perfectly co-resonating in the PRMI when the arms are not locked. See Fig 1, which is the fields in the PRMI using the design PRC length. If I add 5cm to the PRC length, I get Fig 2, where the peaks are about the same separation, but the upper and lower sidebands have swapped sides of the 0 mark. However, if I remove 5cm from the PRC length, I get Fig 3, where the peaks are much farther apart than in Fig 1. This case looks more similar to the data that Gabriele plotted in his elog entry, where the peaks are separated by at least a linewidth. This is not at all conclusive, but it's a guess for which direction we need to move. Obviously doing an actual measurement will be better.
My tummy feelings also agree with this simulation: When we flipped PR3 (the only optic change in the PRC since Gabriele and I measured the 55MHz peak separation in April), since the HR side of the optic is now at the back, we had to push the whole suspension cage forward to get the beam aligned to the Yarm. Conversely, however, transmitting through the glass substrate adds to the optical path length. So, my tummy feelings may be wrong.
Figure 1, PRC at design length, PRMI sweep:
Figure 2, PRC 5cm longer than design length:
Figure 3, PRC 5cm shorter than design length: