John was able to reach my email and send the two photos, so I have written to him in response. The variation doesn't seem to point to just one factory as I found the zig-zag line on early platters by various makers and by Spode -- also the Spode Blue Room patterns from the late 20th century. There are also Spode and Copeland examples of standard willow pattern in my collection that have just the normal diagonal cobalt line without the zig-zag line. I didn't do an exhaustive study, but enough to know that the maker of the willow pattern plate will not be identified from the shards of the border I saw.