Complex sentence: Originally released in 1939 as a metal-cased tube, it was later registered in 1941 as the glass-cased 6SN7GT, during World War II a 6SN7A was developed as a slightly improved version, then also a more rugged 6SN7W for military use.
Split into simpler sentences: Originally released in 1939 it was officaly registered in 1941 as the glass-cased 6SN7GT. During World War II a 6SN7A was developed as a slightly improved version, then also a more rugged 6SN7W for military use.