Lambton,
I think JS gave the explanation correctly, the clue lies in 30T.
Alan is probably looking at the gear on the spindle and the same sized gear after the tumbler reverse cluster.
For his benefit the screw cutting gearing starts from the second of these 30T gears and the tumbler set does only what it says in the name and reverses the drive into the gear chain down to the leadscrew.
It is flanked by identically sized gears above and below to keep the speed unchanged from that of the lathe spindle.
Myford in their literature call the short shaft that the second 30T gear runs on the 'Mandrel'
I hope that makes the table easier for you to understand Alan
Regards
Brian