Alternately you can just mill the cuts into the upper receiver, which is what I did. Came in useful when the plate protecting the valve knocker snapped and I had to replace the screws and the plate.
You may need to do some minor fitment sanding as well... I had to bore out the takedown lugs on the upper to reduce the stress on the lower receiver. It's still snug but at least I can now push the pin out using my finger. Then again I had a receiver from a company known for snug tolerances.
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