I'm awarding myself the Homer Simpson Award for Excellence in Magneto Troubleshooting. Doh!  On the Internet I found the Bendix S-20 Series Magneto Overhaul Manual . Page 17 of the .pdf reads:

"Primary resistance between breaker and ground leads shall be 0.2 to 0.6 ohms. Secondary resistance between high tension terminal and ground lead shall be 12000 to 16000 ohms."

Luckily, my A&E hadn't sent the mag back to the shop yet so I took an accurate measurement with my Fluke and the primary resistance turned out to be 0.4 ohms, smack in the middle of the proper range. I apologized profusely and slunk back to my hangar with the left mag.

But this saga isn't over yet, because I checked the right mag and it reads open at the P-lead terminal. To be continued...