The left part contained Cyrillic letters Ш, Б, М, Н, К, Ы, И in a definite order, with width of each character equal to its height, and the size of a first row character being 70 mm, in the second row it was 35 mm, and 7 mm in the last row.
Bulgarian standard is the same except for the two letters given in parentheses (Bulgarian language does not use Ы, while Ъ, missing in the Russian standard, is frequent).