The markings are Plimsoll Lines, ususally found on the sides of ships to indicate their weight in the water. I have several types of glasses spanning a few ranges with these markings, but I've yet to figure out why. I'm currently assuming they were produced for a shipping line, as I did find a set of carafes with original packaging... but it was an unmarked polystyrene container which would indicate they weren't made for retail.
I'll scavenge through my resources to see if I can find the exact model of decanter.