What do the three letters CPO stand for?

Czech Radio archivist Miloslav Turek helped find and technically edit rare images that were broadcast in 1937 or 1938. In many ways, the problems of the pre-war Czechoslovak Republic were undoubtedly the same as ours today, but there was one additional phenomenon: The inhabitants of that time also considered CPO (cé pé ó) - civil air defence - to be their daily concern. Three letters whose importance was duly respected in the radio of the First Republic.