How's it going guys,

I'm getting ready to build out a Nylaug, and I am confused about the options for the Trunnions. I wanted to buy an aluminum trunnion from 3dprintfreedom or Riptide Rails. In the Riptide Rails description, it says it is best used with the "Standard" receiver. Now I am confused about which is the "Standard" receiver, and if the 3dprintfreedom is the same case. My question is whether or not if its for the SURPLUS stock or the SEMI Auto stock?

Thank you