The British triangle registration mark was between 1845 and 1883....
Then they went to just the Rd number.
The site linked below shows how to date by the number.
Nice dish!!!!
Linda C

Here is a link that might be useful: Registry numbers.

Thanks for the link! This is helpful! I love it and I have a matching one that must be not as old - no triangle or dates / rd. no - just a stamp.

This is the bottom of the one pictured. So if my rd number is 208597 and the date says July 27/93, is it safe to say this is 1893? Any idea how to find out the value? I know very little about this stuff. Thank God for the internet!

According to the table I linked, your dish was made in 1893.
The patent date doesn't mean much...only when the design was patented....could have been made long after it was ptaented.
It's not a "casserole"....don't try to bake in that....it's a covered serving dish.
As for "value"...look at completed sales on ebay for "white china covered serving dish"...or white semi porcelain covered bowl...
At a shop or sale where shipping wasn't a consideration on price, I would say you could get $50 for that dish.
Linda c

Linda, Thank you so much for responding and for the feedback. Sorry for the alarm. I wouldn't dream of baking with it. Another woman told me it was a casserole serving dish. :-)
Thanks again!
Kathryn