Although Matt was able to work around his problem, this problem seems to still be present in 2.7 as well as all 3.x versions. I think I've found the cause of the problem: in 64-bit code, the calling conventions for passing structures by value are different. From this page:

https://msdn.microsoft.com/en-us/library/zthk2dkh(v=vs.90).aspx

I would point to "Structs/unions of size 8, 16, 32, or 64 bits and __m64 will be passed as if they were integers of the same size. Structs/unions other than these sizes will be passed as a pointer to memory allocated by the caller. For these aggregate types passed as a pointer (including __m128), the caller-allocated temporary memory will be 16-byte aligned."

The code in ffi_prep_incoming_args_SYSV (see https://hg.python.org/cpython/file/a1b76c1c3be8/Modules/_ctypes/libffi_msvc/ffi.c#l368) assumes (see lines 399, 402) that all value parameters are always passed inline on the stack - which is plainly not as per the documentation I linked to, for 64-bit code.