Sounding weights were usually very large, several pounds at least. They also usually had a concave depression in the bottom to hold wax so the type of bottom (sand, rock, etc.) could be discerned. Your weight looks like a fishing weight. It would not be able to take a trip straight to the bottom on a string due to current or speed of vessel. A sounding lead had to be heavy enough to go straight down fast in a current and soundings were usually done with the vessel under way and the weight needed to be able to overcome the water drag on the cord in either condition.