To be correct, you should get a .223 or 5.56 NATO elevation knob.
Here is my SIG 551 STANAG mount setup with the Hensoldt Fero scope plus 5.56 NATO elevation knob. I got this SIG mount from a friend in Chile.
Here is the scope mounted on my 551, should anyone care. A SIG cheek riser is needed to do any actual shooting.