Are the receivers machined the same, again, with the exception of the safety?
No. Even with the E which is almost a pre-mil there are differences. The cocked indicator is larger. The screw that locks the carrier group in place is larger. Through in the tang safety and you open up a whole 'nother can of worms. I'm sure that there are a few other differences that I've forgotten to mention.
Thank you all! 
