That appears to be the RA adapter. I've got one if you're in the market.
They've got another adapter that takes AR tubes that may require an endplate but based on the lines on that adapter I'm going to say it's their original fixed m4.
Looking closer at the pic, it would appear that stock has an end plate. The RA adapter for the end plates sits the tube a little higher so I'm not sure if the stock rods would clear the upper.