I'll try to get a photo of one soon and post it. I believe it's supposed to kick on above 50 degrees or so and heat up the element on the choke more quickly, causing it to open quicker. You may already know, but on the older 318s like your and mine, the exhaust crossover passage in the intake manifold is way smaller than on a 360. The spring loaded butterfly valve on the right side exhaust manifold is closed when cold, forcing warm exhaust up through the passage and around the choke, warming it up and causing the choke to open. When that dinky little passage starts to get carboned up, exhaust can't flow through and it takes forever for the choke to open. The rich running that causes creates more carbon which eventually plugs the crossover completely. Every 318 I had needed to have the intake manifold pulled and the passage cleaned out. The only one that hasn't needed it is the 318 on the combine. The industrial versions have a manual choke and the crossover passage is blocked off. As far as your little heater control goes, my folks '73 D100 Adventurer had one, but my '77 D150 and '74 D300 never had one. But, my '77 W100 parts truck has one on its 360. I don't know if the D&W 100s used them because of the stricter emmission regulations that applied to the light duty pickups, which the D150s and up were exempt from because they had a GVW of over 6,100 lbs.
