Latitude C640 doesn't work with PA-6

A customer of ours has a C640 and says it is in the PA-9 family. I believe it is PA-6 family as this is a 70w laptop and perhaps Dell reduced SKUs by going to only the PA-9. The PA-6 won't charge his laptop but the PA-9 will.

1. IS the C640 in the PA-6 or PA-9 family and SHOULD the PA-6 charge a C640?

2. What is the difference between the PA-6 and PA-9 as well as pinouts since the PA-9 has a pin 2 (PA-6 does not)