Caterpillar 22 name

Can anyone help?, a question came up at a tractor club quiz night with reference to the Caterpillar 22. Does the 22 refer to the horsepower or the fuel tank capacity?. The answer given was the fuel tank capacity, but my understanding was all early Caterpillar numbers were based on horsepower until the RD / D series machines came out.

It was either 22 belt or drawbar horsepower.

The 22 being 25hp , i think they used 22 because they had used 25 on earlier 20-25-28 tractors but the specs show 22 gallon tank lol

Yes, Cat certainly got in a mess regarding model numbering with two model 20s and two 15s.
It's a problem they continue to have with renumbering D10 to D11 and, no doubt other examples.