Older Shimano Road Brake Levers vs Newer Ones Quality (SLR vs Super SLR)
The difference between SLR and Super SLR is mechanical advantage: Super SLR moves the cable for slightly longer distance with slightly less force. In principle, the brakes work best when brakes and levers are designed to match (like the configuration you are asking about), but the difference is so small that mixed setup (like what you have now) does work too.