>>> class B1(ctypes.Structure):
...     _fields_ = [("data", ctypes.c_uint8 * 256), ]
...     _pack_ = 1 
... 
>>> a= B1()
>>> x = memoryview(a)
>>> x.format
'B'

In the first case the format is 'B', in the second case the
format is:

>>> x = memoryview(b)
>>> x.format
'T{(256)<B:data:}'

While the latter is probably a valid PEP 3118 format, it's
not implemented anywhere outside ctypes See #3132.