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This is a printable AR-9 lower receiver. Two versions are avalible. The Super version requires the brass inserts from the Hoffman Tactical Reinforcement Kit. While the Basic version does not.
The only non AR-15 part needed is a 3/32" DIA x 1" Long roll pin for the mag catch. The mag catch spring is the same spring used with the AR-15 mag catch. Assembly of the ejector into the lower should be intuitive.
See this page for basic AR-15 assembly instructions: https://hoffmantactical.com/how-to-installing-bushings/