You already stated what it is-Heavy Ball. Here is a little history lesson. It was designated the M1930 Heavy Bullet Type D load for use in M1910 Maxim MGs (the "D" stands for Dal'noboinaya=Long Range). It is a 182gr and in the Maxim has an max range of 4,500-5,000m. It was colour coded to differentiate it from the normal "light" ball round which was a 148gr bullet. The light ball had a silver stripe (like the czech milsurp you can buy) while the heavy ball had a yellow one. After WWII when the Maxims were phased out of service the heavy ball round meant for them was as well and production ceased in the 1970's. As a result, there was no longer a need to colour code the rounds since only light ball was being used. That is why standard 54 ball ammo today has no colour code.