Here is one labeled for GBBR use....and SystemA....
http://www.tokyo-model.com.hk/ecshop/goods.php?id=5231
Now a question for the gunsmith on ASC, how is the G&P AEG receiver thread compare to a SystemA? This is more for my own curiosity.....and also, as strange as it was, the nut I got worked out fine out of the box for my G&P GBBR anyway....