A couple of quotes from the Wikipedia link above:
"The resulting octane rating of TVO was somewhere between 55 and 70."
... "Because TVO has a lower
octane rating than petrol, the engine needs a lower
compression ratio. On the TVO version of the Ferguson TE20 tractor, the
cylinder head was re-designed to reduce the compression ratio to 4.5:1. This reduced the power output, so the
cylinder bore was increased to 85 mm to restore the power.
[2] The
petrol version had a compression ratio of 5.77:1 and a cylinder bore of 80 mm on early versions.."
So the 1920's fuel must have had an octane rating below 70 for a compression ratio of 4.5:1? So that is why the lead was added?