Operation of the Tower mint was always expected to make a profit for the Treasury. Die cost and life as well as production output were key concerns. The use of the mill to make milled coins increased die life but at the expense of thru’put. The improved quality of the coins was also a consideration but there again silver coinage was subject to everyday useage, so any finely made coin on the mill was soon reduced to a worn out silver disc in need of re-cycling into new coinage. Another option was to save money on making the necessary dies. How might this be achieved ?
The obverse die esentially remained the same between 1649 and 1657 with the mintmark sun. In any tight squeeze the mint could look in it’s store of old die and resurrect an old die for current production. After 1657 this option temporarily came to an end with the introduction of the anchor mintmark on obverses. I have seen no obverses where the sun was altered to anchor.
So the obverse die, was a die which could be used in multiple years interchangeably. What of the the reverse? Well the unique design of Commonwealth coinage helped again to make the reverse die very flexible, as well. After 1649, the rev dies were essentially very similar except for the last digit in the date. So a fresh or partially worn die could be used again by just changing one digit in the date. In fact a batch of reverse dies could be made with the last digit of the date missing and then pressed into service as needed. Here are a couple of interesting examples.