Petro - Kursa lernolibro por laboristoj

The 1956 publication referenced is the 8th edition of "Petro: kursa lernolibro por laboristoj" (Peter: a course textbook for workers). It was authored by Eŭgeno Lanti and published in Paris by Sennacieca Asocio Tutmonda (SAT), the worldwide radical-left Esperanto organization.