http://www.thepotteries.org/mark/reg.htm#1842

The link above shows how to read the registry mark. The mark does not tell when the plate was made,, only when the design was registered !! This design was registered in Sept 18th of 1865 with 6 items covered in the registration.!!

https://kovels.com/media/kunena/attachments/302645/diamond3.jpg

If you supply a good close up image of the diamond registration mark, it should be possible to date the plate to the exact date it was produced.

Andrew Jackson
[url=http://www.oldeworlde.co.uk]OldeWorlde Antiques[/url]

Yes,, this is the type they were known for !! The staining will affect any possible value it would have !!