After installing OpenSSL, downloading the pre-built Swig executable, and ensuring the openssl libraries are located in the default c:\\pkg, pip install m2
c:\\pkg
pip install m2
I got lucky - there's an unofficial binary installer in lieu of chandlerproject.org/bin/view/Projects/MeTooCrypto being down:
https://github.com/saltstack/salt-windows-install/blob/master/deps/win32-py2.7/M2Crypto-0.21.1.win32-py2.7.msi