As I understand it, the procedure to revert back to 1.45.531.1 is to apply any x.xx.531.x ROM, and then the OTA update will become available to update the phone to the most current 1.45.531.1 or whatever the latest OTA happens to be at that time...
I'm sure there are folks on here much more knowledgeable than I who will correct me if I'm wrong, but from the reading I've done, that would be the procedure in the absence of an actual up-to-date ROM file.
pfx