Third Period Black Mark 1926-1946 introduced in 1926, possibly to mark the Wembley Exhibition of that year.
Also shows registry mark "REG No 0857" - this registry mark dates from 1884 but was only used from 1926. This mark is always black and was used up to 1946. A round scroll with Celtic decoration and the words "deanta in eireann" (made in Ireland) added.
(Source: https://www.belleek.com/en/us/Date-Your-Belleek/cc-22.aspx)