class FILE_HEAD(ctypes.Structure):
    _pack_ = 1
_fields_ = [('name', ctypes.c_char * 4), ('size', ctypes.c_int), ('Cal_l',
    ctypes.c_double), ('Cal_r', ctypes.c_double), ('Speed_ChL', ctypes.
    c_byte), ('Speed_Pulses_ChL', ctypes.c_int), ('Speed_factor_ChL',
    ctypes.c_double), ('Quantity_ChL', ctypes.c_char * 3), (
    'Description_ChL', ctypes.c_char * 32), ('Unit_ChL', ctypes.c_char * 8),
    ('Speed_ChR', ctypes.c_byte), ('Speed_Pulses_ChR', ctypes.c_int), (
    'Speed_factor_ChR', ctypes.c_double), ('Quantity_ChR', ctypes.c_char * 
    3), ('Description_ChR', ctypes.c_char * 32), ('Unit_ChR', ctypes.c_char *
    8)]
Структура python ctypes имеет неправильный размер байта