In the order they apear from left to right, I know its: #cyls-Plug gap-point gap-dwell-????-Timing. Whats this figure in which most of them are 17-21? As a guess, I would say it looks most like a vacume reading, but your the one with the book.
Just remember that the timing spec is only a guide line that was established when gasoline was a lot different than it is now. My 72 300 6 pings like crazy at factory spec and needs to be backed down 2 degrees. It is a fine line and if I back it down 4 degrees it has a noticable power loss. On this you need a good dial back timing light and check all your advanc specs as per the manual. I have a 1975 Motor manual that has all the information in it which has been reliable except for a typo or error on the valve adjusting procedure for 390's.