ctypes pointers and SendMessage

[Metalone]

I would like to call
windll.user32.SendMessageA(hwnd, win32con.WM_COMMAND, wParam, lParam)
where
lParam represents a pointer to an object.

And also convert this pointer back to an object reference inside of
wnd_proc
def wnd_proc(hwnd, msg, wParam, lParam):

So something like this:
class X(Structure):
    def __init__(self, x):
        self.v = x

x = X(1)
windll.user32.SendMessageA(hwnd, WM_COMMAND, 4000, addressof(x))

def wnd_proc(hwnd, msg, wParam, lParam):
    if msg == WM_COMMAND and wParam == 4000):
        x = X.from_address(lParam)
        print x.v

Unfortunately, this does not work.

Also to my surprise the following does not work:
x1 = X(1)
x2 = X.from_address(addressof(x1))
addressof(x1) == addressof(x2) --> True
but
x2.v --> 'object has no attribute v'
x1.v --> 1

Also this does not work as I expect.
p = pointer(x)
p[0].v --> 'object has no attribute v'.

What am I doing wrong?