Thank you Helicopter-Redeye. Do you have any pictures of the extender tanks by any chance?
Bravo 73, the appropriate passage from the same page in the Astro/Raven handbook is:
Caution
If mixture control is leaned at high altitude, be sure it is pushed back in before descending to lower altitude, otherwise, engine may quit.
Revised 5 Nov 1999, so may have been superceded