The Title of 20th century Bulgarian Rulers

The text on Bulgarian stamps from the beginning of the 20th century (here Mi #112) translates to "Tsarstvo Bulgaria". The ending "-stvo" means "-dom" like in "Kingdom" - or "Tsardom". A direct translation would then lead to the title "Tsar" of the Bulgarian rulers. However, considering the constitutional situation in the country, it would be equivalent to use the title "King" in English. Thus, both titles can be found in litterature.