The rear part of Yugo milled receiver is the "trunnion". The buttstock fits into the hole you are talking about in the same fashion as stamped Yugo AKs. This is done with a long bolt that runs from the butt of the rifle to the rear of the receiver, kind of like a Lee Enfield.
Couple of Yugo's I've built. Top picture is a Yugo milled Tort-Tort RPK bottom is stamped Recreator receiver for comparison.
In my opinion I'd look no further than Tort-Tort for your build. I've build on a number of his receivers and they are top notch along with his customer service.