Man that is looking flawless!
Can I ask about the technique you're using to get such smooth joins with the putty?
Mystery Part-
MPC Snowspeeder Aft 'Gun/Truster' part?
http://2.bp.blogspot.com/_GX2mRgZEogY/S769XElp3SI/AAAAAAAAAQI/mLN6SoDhH2M/s1600/cfx_s1.jpg