Now that your question is answered, can you tell me where you found a BCG of any kind for sale? Looking for one myself.
OOPS! Just read your post where you back ordered it.
BTW, I can't imagine why anyone would tell you you need a back up BCG. Not much there to break or wear out. I could see possibly needing an extra extractor with spring and pin and maybe even a gas key after a very high round count but the carrier itself is pretty much a solid piece of steel. I have never needed any spare parts due to wear or breakage in 36 years of shooting M16s and AR15s. Now I have lost a spring here or there.