In the First World war, the Serbian Army used oval cap badges with the Serbian national colors of red-blue-white, with red enamel in the centre, blue in the middle area next to the red, which in turn was next to the outer colour of white. Enlisted men wore the plain, red-blue-white badges, and officers wore the type with the royal cypher of King Peter I, which consisted of a Serbian Cyrillic 'P'(that is, 'П'), and a crown above it and a Roman numeral 'I' to denote the First, ie. King Peter the First, and the other type has the Serbian national, heraldic eagle on it. There are numerous, slight variations on the design of the royal cypher on badges of this time, and also the design of the eagle, as manufacture of the badges was not from a single source. Also, for enlisted men, it wasn't uncommon for there to be infantry units without cap badges as this might depend on the availability of their supply at the time.
Serbian Army badges worn by enlisted men and officers.