Nick - I stand corrected sir. I missed the '39 listing with the 3113455 number.
Great pictures and info BTW!
I agree with your surmising the "39K" is most likely November 1939 build date, which would coincide with the 1940 Model year, I think.
A closer look at your pictures shows it's already been to at least one radiator shop since new. The Ft. Collins Radiator numbered tag above the upper radiator hose neck attests to that.
I'm with Kevin on attempting a self repair. I would suggest that when you start trying to clean off the paint for soldering or applying the Marine Tex, be extra gentle with a hand operated wire brush using light pressure. The brass tank could suffer damage if you get too aggressive with the wire brush.
