Hi Pete,
lol maybe you don't get out much either , many continental glasses both old and new are marked in various ways , you have measured the contents and revealed it was 300 ml approx, so the mark obviously equates to 300 ml or 6/20ths of a litre, why it was marked this way and who for I dont know ( I am sure no one asked for 6/20ths of a litre please) , a bar maybe and it was the largest wine measure , i think you have the answer as to what the mark means ,which both Paul and Misha also mentioned.
cheers ,
Peter.