Manifold heat control valve.

There were 2 types of counter-weights. My 1964 slant and 1965 273 have round weights. My 1969 slant had the sheet-metal weight you show. I thought it was a year thing, but if you have the 1965 manual, perhaps slants got the sheet-metal then (I'll check my manual at home). Seems to me the round weights are a better design. Long ago, in my 69 slant it was rusted stuck and I recall the innards were mostly gone, so I installed stainless bolts & nuts in the holes. Didn't make winter starts any different in Atlanta.

I infer that a heat riser isn't needed w/ MPFI, which I plan someday for my slant. That is because I see no such provision in my newer Mopar engines (2.4L, 3.8L), nor in a Magnum V-8 intake I have. I think its main purpose was to avoid icing under the carburetor in the winter, though might have helped idling in winter after 30 sec running (time to heatup exhaust).

Before re-installing, be sure to read the excellent post at www.slantsix.org on installing the manifolds. The triangle and cone washers install the opposite of intuition, and if you do it wrong the exhaust can later crack. Best to coat the inside clamping bolt w/ anti-sieze since so many snap off there when removing. I used stainless bolts, but grade 8 might better (Ace has nice selection).