Mr. Obvious,
This discussion spun off the discussion below about magnetic engine heaters. You weighed in about tank heaters.
I agreed with you about the effectiveness of coolant heaters.
It does not matter if it is an N, a later Ford,
a John Deere, Farmall or a Kubota. The principle of cold weather starting is the same.
Cold temps reduce the volitility of the fuel air mixture making the engine harder to start.
Heating the combustion chamber increases volitility whether you are running gas, propane, diesel, kerosine or even wood gas.
The most effective way to heat the combustion chamber is to heat the coolant that surrounds it.
Heating the combustion chamber is a more effective cold weather starting aid than heating the oil with a dipstick heater or magnet heater on the pan - especially nowdays with multi viscosity oils.
I see no great advantage to your type of coolant heater over another.
Perhaps your tank heater has some advantage when it comes to circulation but convection heating via a block heater or hose heater is nearly as effective.
For more comparing, a tank heater is probably the least energy efficient of the 3 types.
Compare your external apparatus to the photo of my block heater. Grab your tank heater while it is on and I'll bet you won't hang onto it very long. There is a lot of heat being dissipated via the tank and hoses Before it ever gets to heat your coolant. A block heater is inserted directly into the coolant so there is very little wasted energy. I suspect a lower hose heater falls somewhere in the middle of the two.
Also, look at how clean of an installation a block heater is. A lower hose heater is only slightly less so.
Both are much less likely to be torn off the tractor by brush or something than a tank heater too.
I installed a tank heater on my 69 Rambler back in the 70s. It worked good. But I doubt I would ever put one on a tractor. I've just seen too many tank heaters that look like the ones below to ever want one.