Sorry I can't be of more help, since I don't have real detailed info regarding Bulgarian aircraft during WWII...
I do know that the B.534 numbered 78 according to research by Bílý, Miroslav and Jiří Vraný.
Thier book, Avia B.534: Czechoslovak Fighter ISBN 80-86524-15-9 is a very accurate resource.
The list I do have (not at all complete), shows the Royal Bulgarian Airforce operated these aircraft during thier Axis years:
Arado Ar96B-2
Avia B.534
Avia B.135
PZL P.11
Dewoitine D.520
Dornier Do17
Kaproni Bulgarski KB-11 Fazan
Arado Ar65
Heinkel He51
Messerschmitt Bf108
Messerschmitt Bf109
Junkers Ju87
Junkers Ju88