If you flip the black probe onto the tip and the red probe onto the ground you should read 9.3v. If this is the case you have reversed the polarity and quite possibly have fried the preamps opamp. Your one spots center pin is negative and the shield is positive. You will need to rewire the plug for the power jack to the guitar so that the positive is on the tip and the negative is on the ground. You have the equivalent of putting a 9volt battery in backwards. If you powered this with the polarity reversed as it is, you most likely now have a bad preamp.
You did say that your other guitar is wired identical to this one but i am not sure why your other guitar would work with the positive feeding to ground, something has got to be different with it?
Disclaimer: I make and sell Buffers. I also sometimes Modify, Build and sell preamps and amplifiers. My opinions are not intended to be sales pitches. I am a one man shop and prefer to spend time with my wife and family, i work full time on call and love to spend my spare time doing other things. I only make, modify or build things for those that seek them.