The Prince50 fixed-mag kit is another fixed-mag kit, which uses an allen screw on an otherwise normal magazine release button. Therefore, it can be used as a detachable mag when out of state.
The B-15 bullet button is a bullet button which works with the Prince50 maglock. This allows the rifle to work as a bullet button equipped weapon in California, or a normal drop-mag rifle outside of CA. The same vendors carry this, as all the other kits.
The Bullet Button is a fixed mag kit that allows the magazine to be dropped with the use of a tool. A bullet or ammunition cartridge is considered a tool.