Thanks for the helpful info. But I have one very minor quibble. I recently found an external battery pack that would appear to be part of the original 645 system. But the chart above says the original 645 didn't have an external battery pack. As you can see in the photos below, the writing says "Pentax Remote Battery Pack 645" and the finish on the grip is identical to that of the original 645, which is distinctly different from the finish of the 645n. I tested it out on my 645n and it works fine.