in the beginnings of roll film, zeiss started with the 520 series and numbered the formats successively. the started with 4.5x6, just plain 520, no suffix and continued with 6x9, suffix /2. they continued numbering, there was for instance 520/14 (5 x 7.5cm), 520/15 (6.5 x 11cm), 520/16 (6 x 6cm) and 520/18 (3 x 4cm).
when they realized that their internal numbering led to confusion, they added capital letters, 3 for 120 film, A, B, and C (and D for 116/616 film), A=4.5x6 or 520, B=6x6 or 520/16 and C=6x9 or 520/2. By 1938 the 521 series was launched, a 523 series in the early 1950s and a 524 series added an uncoupled rangefinder, it was called Mess-Ikonta.
Models with coupled rangefinder were called Super Ikonta. The series started with the number 530. Then the numbers get a bit mixed up. There was a 532 series, a 533 series had a selenium meter added. The last models, built from 1956 to 1960, were the 531/16 (Super Ikonta III) and the 534/16 (Super Ikonta IV) with a non-coupled selenium meter. Why this new design got an inferior number (531/16), I don't know.