Looked through my pictures of Dearborn and NJ 68 (only) examples. From the limited number of 68 Dearborn picture I have I don't have one with showing/using the electro magnet
For 68 NJ the results are mixed and I can't see a pattern but we do have many more examples since the production would include Shelbys also. Guess I'll assembly a group of 68 NJ examples for all to see (Deaborn isn't going to provide anything more than what I've provided in words at this point from my source) and post
Bottom line as I see it this moment is that we're off chasing when each version was used but I think we all agree on what finish each version was originally 
Something I was thinking about regarding the clutch differences seen (and any patterns we may be trying to establish), the early version (brushes style) clutch could easily have been updated to the later version (or vice-versa) during any service procedure. I would be willing to suggest that a Ford service garage would typically sell what inventory was on the shelf first and unlike today's Parts People, these REAL parts People of the Old School variety, KNEW what updates or current inventory stock fits what cars WITHOUT relying on the MPC to tell them. These guys just walked out to the shelf and got the part, they rarely even needed to look the part up.
I agree with your strategy but would like to add because of others that may be reading is that only the very earliest of 67 production A/C cars would use the brush set with the C3SA engineering number. I actually have never noticed the C3SA brush set used before on a 67 production but it may be because your car is early production and Ford hadn't switched over yet. The vast majority of 67 Brush sets are of the C7 variety. The C3,C7 brush sets can still be found in NOS form occasionally. The C8 is much more common relatively speaking.
I would have no way of knowing that the C3 brush set is ACTUALLY original to my car or consistant to what is found on low-mileage unrestored for an early November, '67 SJ example. I can only say it is what I found during disassembly. (Refer to the comment above) My car was driven over 120,000 miles and the air conditioning would have been used A LOT in the Mojave Dessert in the first 10 years before I got the car in early 1978. The AC system was still functioning as a daily driver in the Palm Springs area well into the mid/late-80's before I quit using it due to an expansion valve defect. This AC system had MUCH use and as we know, the brushes rub on the pulley clutch even when the AC is not being used (wearing down the brushes). The C3 Brushes are now (perhaps, 'again') pretty much worn out at this time but it isn't impossible they are the 2nd set or 3rd set or whatever.
