Complex sentence: Commemoratives followed in 1891 for Hong Kong and Romania, then in 1892 and 1893 a half-dozen nations of America issued commemoratives for the 400th anniversary of the discovery by Christopher Columbus.
Split into simpler sentences: Commemoratives followed in 1891 for Hong Kong and Romania. In 1892 and 1893 a half-dozen nations of America issued commemoratives for the 400th anniversary of the discovery by Christopher Columbus.