The Bulgarian stamp has been issued however Bulgaria was not present at the Expo 58. Pure technically, we could say that this is not an Expo 58 stamp but more a stamp for thematic collectors.
The stamp has been issued at the end of the Expo 58 happening: 13 October 1958. The stamp has been issued in both perforated and imperforated state. No information about number of stamps printed nor about the engraver etc.
The stamp has been issued in sheets of 50 stamps at once. See also below.
Sheet of 50 perforated stamps:
