When it is not only the only piece known, but when there are no other denominations known in the series (until another is found 😀), either.

The best part is that it was on eBay for several months, undetected... (where even I passed over it) only to find it by stumbling over it by including one single term in a search looking for something else... and it wasn't even via eBay. What a shock that was to see it pop up while not even knowing it or any other plaque from the series existed.... and selling for a very inexpensive price.

Yes, I am sure there may be another or even many more, but thus far this is the only one and I do not see it in some of the World's most complete collections.