As I understand it, and someone smarter than me please correct me if I am wrong, DRILLED pretty much means that there are holes drilled for bulkheads(no overflow). RR indicates that there is an overflow of some sort (internal or external), and the bulkhead drains are in the overflow.
But this brings up a question, what would one call a tank like mine for example that has an overflow, but is also drilled for a CL? Is this a DRILLED/RR?