Bulgaria: Class 15

Even if the Bulgarian State Railways (BDZ) were already using Class 52 units during the Second World War, they were rented from German Imperial Railways (DRB). By September 1944, 125x units were in use in Bulgaria. Most remained behind after the German withdrawal from the Balkans and the Red Army claimed them as spoils of war. 85x units were leased back from the Soviet Union (USSR) and are acquired by the end of 1946 and are registered as Class 15.

Sources: https://de.wikipedia.org/wiki/DR-Baureihe_52#DR-Baureihe_52.80

20x Class 52 units are rented from German Reich Railways (DR) for one year and are bought after the end of the year.

Sources: https://de.wikipedia.org/wiki/DR-Baureihe_52#DR-Baureihe_52.80

30x new units are acquired:
- 20x ex. Class 550.0 units are acquired from Czechoslovak State Railways (ČSD);
- 10x ex. Class 52 units are acquired from German Reich Railways (DR).

Sources: https://de.wikipedia.org/wiki/DR-Baureihe_52

140x units are acquired from the Soviet Union (USSR) between 1961-1964.

By 1965, the total stock was comprised of 275 locomotives, making the Class 15 series the largest steam locomotive series in terms of number in Bulgaria.

Sources: https://de.wikipedia.org/wiki/DR-Baureihe_52