Welcome to ak-builder.com!
The Polish Underfolder is the most popular underfolder parts kit and this was the first specialty flat that I had perfected by modifying existing flats in 2004. Now they are stamped out specifically for the Polish Underfolder from raw steel.
Reccomended rivets and center support (not included with flat)
To use these flats you will need to have a 5mm and 7mm reamer. The physics of bending sheet metal make it next to impossible to have a bend close to a hole without distorting. To solve this the holes on the flat are undersized and need to be reamed to size after bending. Look at the receiver flat instruction page to see more info.
The rails that will come with the flats are pictured below. They have mag stabilizers on both rails to keep the mag from tilting and causing feed problems. Before installing the rails you will need to trim the mag stabilizers to match the caliber of flat you are using.
This item is made in the United States and counts as a 922r compliant part.