Hi Trey,
I do believe this is a European bottle and the 20 refers to the # of liters volume. You might test my theory by adding measured amounts of water. 20 liters is a bit over 5 gallons.
As for the "EM" I am not sure. It could be a manufacturers mark.
Demijohns are widely collected - I once collected them and might have had 100 examples or more at one time - you can safely say this is worth in the $50-100 range to the right person.