It's generally thought to have been a nine-series set of cards, thirty-six cards in each series. Perhaps the confusion sets in when
collectors introduce terms, such as low, semi-high and high to describe sequences in the numbering of the cards. The so-called "high-number series" is supposed to have started with the Mantle card as #253, however, as can be seen in this image of a partially cut 1951 Bowman sheet, Mantle was obviously not the first card on the sheet, and thus the high-number differentiation is an arbitrary standard, not imposed by Bowman or the Zabel Brothers printshop. Interesting speculative article on the 1951 Bowman printing sequence here:
http://pjdenterprises.com/baseball_c...man_print.html