First off, I want to thank everyone for all of the wonderful suggestions they made. All of them were quite good and helpful. Before I had read your suggestions however, I went at this problem with a thread file, honing stones, and lapping compound. After several hours of work, I was able to get the rear bearing race to smoothly thread back on the main shutter shaft! Adjustments are still needed to center the rear bearing support to the front bearing, but victory nonetheless. Still puzzled by why it came off easily but wouldn’t go back on. But onward and upward!