It would appear that serial numbers went on frames at most gun companies as the frames were produced. Most would be finished and shipped sequentially, but there could always be some delayed by engraving or other special features, or simply being misplaced.
The BATF considers the date of production to be the date the frame was made. I guess it's conceivable that a company like Colt could find a batch of old pre-1899 Single Action Army frames that were lost in a warehouse, and finish them today, and then legally sell them by mail order without an FFL transfer just like any muzzleloader.
