Small Ironstone Platter

This small (approximately 8" x 5") platter or underplate is marked on the back with an impressed rectangle with "ironstone china" inside, a small impressed mark above the rectangle, 5 small blue dots, an H, and some (pattern?) numbers. I think it's 19th century. Does anyone know who made it?

Thank you!
Janice

 

Found some plates, but no ID: https://www.ymodernnews.com/index.php?main_page=product_info&products_id=686746