This next post is just me having a bit of fun with some supplementary info I found today.
If it can ever be validated that this plate was originally of plain form, it opens up an interesting possibility.
According to a previous post by Silvermakersmarks, the date mark on this plate would have been in use at the assay office between 29 May 1704 until 28 May 1705.
There is a Jewel Office warrant book that documents an order in January 1705 for 32 plain dishes for the Queen's service:
Jewel Office Warrant Book: January 1705, 26-31
26 January 1705:
Subscription by Treasurer Godolphin for the execution of a Lord Chamberlain's warrant of the 8th inst. to the Master of the Jewel House to make 9 new covers for dishes and 32 plain dishes for the Queen's service at the request of the Board of Greencloth : to an estimate of 260l. Warrants not Relating to Money XVIII, p. 403.
This can likely be interpreted as an order for 32 plain silver dinner plates (as opposed to meat serving dishes, etc.). An order of this size could have been for replacing missing/damaged plates and/or to extend the Queen's service for accommodating additional guests.
As we have previously established, dinner plates were likely sent to the assay office as flat discs and completed upon return to the silversmith. The date of the order, 26 January, would have allowed just over 4 months for the Royal goldsmiths to deliver 32 flat discs to the assay office (before 28 May 1705) for the Queen's plates to have received the same date marks.
Now for some back of the napkin math:
The available warrant books record that between 26 March 1704 and 25 December 1705 (around the time this date mark was in use), approximately 14,000 to 15,000 ounces of new white plate was delivered to the Jewel Office. This would have been comprised of candlesticks, serving dishes, dinner plates, flatware, etc. most of which would have been designated as indenture plate.
Assume that nearly half, or 6,912 ounces was comprised entirely of
plain dinner plates (dinner plates with gadrooned or molded borders would be separate to this allocation). With an average weight of 18 ounces per plate, you arrive at a total of 384 plain dinner plates.
If 32 of the 384 plain dinner plates are designated to the Queen's service, this would give 1 in 12 odds that the plate was the personal property of Queen Anne.
Obviously if the Queen's 32 plain dinner plates with 1704-1705 date mark are already accounted for in a museum or collection then the probability quickly drops to zero.
Calendar of Treasury Books, Volume 19, 1704-1705
Calendar of Treasury Books, Volume 20, 1705-1706