The 20 stands for the gauge steel used!
It was a US government requirement that all manufactures stamped the gauge of steel used, the amount it held and the year.
There is some that state 5.25 or 5 1/4.
I was told this by the king of Jerrycans Philip Lindsay of Germany.
He has written many articles on them a good one is in Army Motors 115.
He also has a huge collection.
Yes I don't think they give a rats about Liters then because they still don't want to know about the metric system. And they say we a behind the times.
Hope this helps.
Cheers
Jeff