The black ‘x’ on white background was Bulgarian (1941-44) while the ‘Yak?MiG??’ is the Avia B.135 (Czech design)… from http://www.aeroflight.co.uk/waf/bulgaria/types/avia.htm

“Avia B-135 Lyastovica
Twelve B-135 (AV-135 in Bulgarian literature) delivered in crates by rail Feb 1943. Assembled from Czech-made parts by DSF at Lovech in the summer of 1943. Last example test flown 28 August 1943, following which all the aircraft were officially accepted by the VNVV. Planned licenced production of 50 a/c as the DAR-11 Lyastuvka (Swallow) later cancelled due to the lack of Czech-built engines. Used by the Fighter School at Dolna Metropolija for the fighter-training and fighter-interceptor roles. Withdrawn from service by end of 1944 and stored in the open at Dolna Metropolija. Last few airframes used for air-ground gunnery target practice in 1946. Military type number 7057.”