The codes for the blanks works like this, example "Purglas 400/3" the first number refers to the length of the blank i.e 400cm and the second number "/3" to its intended casting weight. You can multiply this last number by two as a general rule of thumb to get the designed casting weight and in this case a 400/3 should handle 6oz perfectly but you can also use a 5 or a 7oz.
Blue Marlin made the codes easier, the last number is the designed casting weight, so for a 6 oz sinker you would buy a 400/6. To confuse the whole matter even further, they will put the total rod length on the factory built rods instead of the blank length, so now you are looking at a 430/6 rod which is actually built on a 400/6 blank. But they do not stop there, they also have 3+1, 5+1, and 7+1 blanks, the "+1" refers to an extra layer of carbon wrapped on the blank.