From press announcements we now know that the Dixon, Phillips & Co partnership began in 1839. So if the copper plate was engraved by Pontifex and Stiles for that partnership, it could only have been made in that year. And if it did start life at the Garrison Pottery (Dixon's), it likely would have found its way to Moore's and/or Scott's after 1865, when the Garrison Pottery closed. But there are a couple of 'ifs' in this paragraph.
The Sailor's Farewell
The Sailor's Return
The Flag that's Braved a Thousand Years
Ball's has an already murky reputation as a pottery that printed items with other potteries' marks. But forging a mark would take them into a whole new league. Could the Ball's family inadvertently have donated evidence of forgery to the Sunderland Museum? We may never know, but we owe them a huge debt of gratitude for saving the copper plate.
One thing that might settle it would be to find an 1840s or 1850s Dixon item with the Ball's transfers. However, in many hours of searching over recent weeks, I haven't found anything from Dixon's with these transfers.
Some last thoughts
But with regards to my thoughts of recent weeks, nothing much has changed. We already knew that the Scott versions of these transfers were likely used in the 1860s, because of similar plaques with dates in the Sunderland Museum (see below). And yes, that's the period when the brown-bordered plaques were produced. The 'Scott' versions of these transfers never appear on Moore's items. However, the 'Moore' versions of the transfers do appear on Scott's items, likely because Scott's sent them to Moore's for decoration.