Yes it is...
The limiter (plug) is the rod with the attached round cap. The cap inside the magazine tube with the round hole is the actual spring cap. The additional cap with the "turned out" section is a "keeper" type ring to retain the limiter in the magazine tube. It is actually not required when the limiter is not in use and is probably there just to keep from losing it. I keep mine with my limiter rod.
The "keeper" ring isn't round but slightly oval. The long direction of the oval is the same distance as the inside of the magazine tube. The short direction across the keeper is the same dimension as the entry diameter of the magazine tube. The end of the magazine tube has a lip on it that will hold the keeper inside when it's laying flat. To remove it, push the assembly into the magazine tube with your finger and, using the pushed out tab for leverage, tilt the keeper about 30 degrees to allow it to pass the lip on the magazine tube on it's way out.
To install the limiter, insert rod end of the limiter into the hole provided in the magazine spring cap until it reaches the little clip that's attached to the rod. Then push it into the magazine tube until the flat circular end has entered the tube. Tilt the keeper about 30 degrees and insert it into the tube on top of the circular end of the limiter with the little pushed out portion facing outward. Push it inside and let it rotate to a flat position. When you release pressure, the long direction of the oval will keep it inside the magazine tube.
To disassemble, push the assembly into the magazine tube with your finger and, using the pushed out tab for leverage, tilt the keeper to allow it to pass the lip on the magazine tube. Once it's past the lip, the limiter will follow - BE CAREFUL, SPRING IS UNDER PRESSURE - DO NOT LET IT FLY OUT AND HIT YOU!!!
Hope this helps...
Mikey