It is a different part (think about it, the lever would be different).
I would hang onto the gun and get the lever. If they are sold out everywhere, call HK and they will hook you up. They are all reversible, thats why all USPs have that characteristic white line on both sides. You can also get an ambi set that has it on both sides.
While you are at it, I would recommend getting the extended magazine release, it makes a huge difference.