Going off some discussion we had at lunch today, here is my current knowledge of the state of cavity lengths. 

Acknowledging that Koji changed the sideband modulation frequency recently, the ideal cavity lengths are (to the nearest mm):

• Lprc = c / ( 4 * fmod) = 6.773 m
• Lsrc = c / ( 5 * fmod) = 5.418 m

We when last hand measured distances, after moving PR2, we found:

• Lprc  = 6.752 m = 2.1 cm short
• Lsrc  = 5.474 m = 5.6 cm long. 

However, when I looked at the sideband splitting interferometrically, I found:

• Lprc = 6.759m = 1.4 cm short

This is only 5mm from the hand measured value, so we can believe that the SRC length is between 5 and 6 cm too long. I'm building a MIST model to try and see what this may entail.