Shutters were on all M tractors equipped with kerosene/distillate engines. They were used to help the engine get up to operating temperature. They also had a different manifold as well as a small gasoline starting tank under the hood. Most people do not run kerosene anymore due to the cost.
You have come to a great spot on the internet - lots of great people here who are knowledgeable. Check out my YouTube channel as well - last year I did a complete engine overhaul on a salvage yard '47 M I picked up and brought back from the dead. Lot of issues that motor had, but it runs like a top now!