It would only mean that some of the '1551' coins could have been issued in Jan-Mar 1552. It would still be the case that if the Southwark mint was closed in July 1551, the fine silver would all have to be Tower if the Oct 1551 date is also right. The 1551 'tun' crowns are rarer than the 'y's, suggesting the change over (whether of mint or just mint mark) must have been Jan/Feb 1552.