Mine was built on a .223 by Patriot Arms. (now American Precision Arms)
They did not cut out the indentations in the magazine. At the time, they said they typically build Whispers on a 7.62X39 gun as the mag does not require modification. They charged $35 extra to mod the .223 mag to accept Whisper cartridges. I have since bought an extra mag to modify like they did.