OK, thanks Peck. I was a little unsure about whether or not there was a distinction. I always understood marbling to be the exactly parallel lines of slightly different colour, often running diagonally across the face of a coin. Oddly enough, by way of example, both my 1926 pennies have it. The EF OE, and the barely fine, ME. Not really the same as the 1953 coins, but similar to the 1919 penny & 1921 Italian coin, shown below. Maybe slightly more subtle. As you say, it is possibly the metal content