By glancing at your pictures I don't see anything that doesn't look like regular equipment on a 1931 "GP". There could still be something different about your tractor though. Your one picture shows the fuel tank. Does it have two round holes in it, or does it have a small threaded hole and a larger oval for the fuel? This is one of the changes that happened at 226402 according to the parts books. That's not to say it didn't happen to some earlier tractors.
The most evident changes at 226402 were made to the cooling system and fuel tank. The radiator was made taller to increase cooling capacity. The early radiator sides have a C257R casting number and the sides after the change were C1910R and C1911R. The core and other associated parts were also changed. When Deere increased the bore of the tractor to 6" they must have found out that the cooling system was not up to the task with the extra power and the elimination of the water injection system.
The fuel tank design was, in my opinion, a change made for safety more than anything. The tanks previous to 226402 have a small threaded cap for the gasoline portion of the tank and a large oval opening for the main fuel tank. The main tank takes the same cap as the radiator. This designed started with the first "D"s that came out. I believe the oval opening was to allow fuel (kerosene, tractor fuel, etc.) to be poured out of a bucket without using a funnel. The new fuel tank had the familiar round caps that were used on all of the later tractors.
The most important addition to the fuel tank was the addition of a heat shield like one of the previous posters stated. This consisted of two pieces of sheet metal with and "air gap" in between them spot welded together. This shield sat underneath the fuel tank. Sandwiched between the top of the shield an the tank was a sheet of asbestos insulation. This whole assembly was to prevent engine and manifold heat from boiling the fuel in the tank which was a safety and performance concern. I have personally witnessed an early 6" bore "GP" without these updates, boiling the gasoline in the sediment bowl. Talk about scary! The tank on the early tractors sits very close to the manifold and engine and working the tractor hard would tend to have this effect.
Other minor things were also changed at 226402 but they are mostly internal such as the pistons. One other thing you might want to check is the casting number of your final drive housings. This is located right below the brake drum. The housings were changed to the final design at 225841 so that is another possibility for an update to your tractor.
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