The drive end of the starter was coming off the field frame easy enough but was taking the armature with it. I didn"t see how to seperate the two and was afraid the brushes were going to pop out - so pushed it back together and took it to the shop. Will have it back tomorrow.
What I understand from the shop owner is that the so-called "12 volt brushes" are smaller than the 6 volt brushes in order to reduce starter current and lessen the force from the drive against the ring gear. He feels this results in less wear and may prolong life of the drive gear. Seems like a reasonable argument when using a 6 volt starter on a 12 volt system.
Will let you know how it turns out. Thanks for your help & suggestions.