js5020, I'm going to disagree with you about "C" vs "L" tractors. I think the most defining feature of a "C" tractor is the change in the forward/reverse control. The "L" model tractors have the HI/LO and Forward/Reverse lever on the same bolt through the handle, which I can see in the pictures above. The "C" model tractors moved the Forward/Reverse handle away from the HI/LO lever, mounted separately, and about 3-6 inches closer to the handle. Further, Gravely reversed their traditional "Push the lever backward to go forward" so that you now pushed forward to go forward,
With the slanted plug, the engines are pre 1967 or whenever it was exactly that Gravely switched to the "C" model production. The fact that the carberator has the air intake facing forward also indicates pre "C" production. I think the cannister air filter indicates later model "L" production. I think the official timeline is: 5 hp from start of production till around 1963??, then an increase in rated hp to 6.6 until around 1967? when two changes were made, the spark changed from slanted to straight up and down, and the air filter location moved from under the hood next to the cylinder to out on the handlebar, and the carb air intake moved from facing forward to facing backward. That configuration was rated at 7.6 hp.
I don't know all the ins and outs of the air filter, but early models that I am aware of used an oil bath filter. At some point, (MAYBE in 1963?) that filter was changed to a pleated paper type that still needed to be oiled and the mounting location was left unchanged. Then in 1967, the air filter was moved to mount on the side of the engine over the carb, and changed to the same style filter used by most Kohler engines of the time.
The oil system provides some clues as well. Again, I am not familiar with all the nuances, but there was an early low pressure, then a high pressure but with only partial fitration, then finally high pressure full flow filtration. Full flow filtration was characterized by a spin-on automotive type filter (Fram 8A, but the shorter versions like maybe a 43? fit more easily into the space.) Non-full flow was characterized by multiple oil line connections directly into the filter. I have a hunch that tractors like that were strong candidates for conversion to a spin-on type filter, as I think the multi-connect style filter is NLA.
Anyway, check for a plaque about 2x3 just to the right of the gas fill cap. We'll know more when we know the serial number.