This is the back side. The pilot badge is named and dated 1928, but I can say nothing for the observer badge. Just the period can be framed, from a couple of years after the end of WW1, until 1935-36. More likely 20s, being so extraordinary. But who knows!
What they flew: right after the WW1, all Bulgarian Air Force aircraft were cut to pieces and the country had no right to have an air force at all. So in the next 15 or so years, it existed more or less secretly. The first aircraft was a WW1 Fokker, hidden in a village to evade being cut. It was fllowed by 2 machines, built from the cut pieces of WW1 planes. In 1924-26 were delivered not large numbers of Potez, Henriot, Cauldrone, Bristoll aircraft, also 2 Macci flying boats. Bulgaria could purchase only from the WW1 winners, only unarmed aircraft and with very limited engine power. In 1925 started the production of Bulgarian aircraft, various models, mostly training. Only in the 2nd half of the 1930's some modern aircraft were delivered and the number increased. In 1937 the Air Force, which existed anyway, was officially restored as a part of the Armed Forces and was given battle flags.