Itsu is short for Itä-Suomen Sähkö, the Eastern Finland Electricity company. The company was founded in 1945, after World War II.
Kaarlo Hildén became CEO in 1961 when the company was bought by Idman. Itsu Oy went bankrupt in 1966. Idman oy was finally dissolved in 1987.
Many thanks to Cyril for the catalogue.