1965 Factory Air Pulleys and Belts

Model K is indeed the '65 underdash dealer air made by Chrysler's Airtemp division.
Model 954 is the '65-'66 factory heat/AC system.
The roller (belt backside idler) was generally present only on the bigger cars, whether you had a six- or eight-cylinder engine, and did a good job of improving belt wrap around the clutch pulley and eliminating belt slap. Your theory about the add-on P/S is plausible.