excerpted from don's post:

    1st&2nd digits: 'machine' number or movement family
    3rd digit: indicator of amount of 'special adjustment'
    4th digit: display type indicator


so following their current numbering convention,
the reason for the different designations is explained:

"7S" base Caliber
"2,3" degrees of "adjustment" (in this case, the addition of the plate and 2 jewels)
"5,6" display indication (in this case, date-only or day-date)





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