Dave,
I got the 2 knobs on eBay. Don't be scared.
Here are a couple of examples in Bunns B series book.
One setup has only 2 knobs and the other (route van)has 3 knobs with a different looking AIR knob. The 4th hole is plugged.
Then I saw this part on eBay last week but passed it up because of the price. It's only a 3 holer.
http://cgi.ebay.com/ebaymotors/NOS-1940-41-42-46-Plymouth-Dodge-Heater-Truck-Mopar-rod_W0QQcmdZViewItemQQ_trkparmsZ66Q3a3Q7c65Q3a13Q7c39Q3a1Q7c240Q3a1318QQ_trksidZp3911Q2ec0Q2em14QQhashZitem220371320533QQitemZ220371320533QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories
Rout Van Dash