this is a hybrid frame that uses XD9 slides and internals with Beretta 92fs/m9 magazines, mag release and grips.
to complement this build you need
-xd9 full size lower parts kit. (minus mag release)
-Beretta 92fs/m9 magazine, mag release, 4x beretta grip screws and grips (printable grips are included)

xd9's have self contained rails and don't require after mark ones to be inserted.

the xd9 grip safety is not required for the build to function and can be left out, however without it inserted the trigger slaps your finger wile firing. There is a pocket for the grip safety that can be dropped in from the top first before finishing the slide. the grip safety spring can be omitted if you want, if you want it installed id recommend making a short roll pin of some kind to hold it in place while the grip safety is being inserted.

m9 magazines are about $15 at army surplus stores.