First, install the magazine catch into the receiver as shown.
Next, install the magazine release spring as usual, and the Prince50 mag lock over that.
Thread the magazine release onto the mag catch, just as you would for a normal detachable magazine release.
The Prince50 mag lock. Note the small threaded hole above the normal mag catch hole. For the B-15 installation, you need to ensure that the small hole is on the top. For a conventional Prince50 installation, the small hole can be on the top or the bottom.
A close-up of the Prince50 lock. When it's used as a Prince50 mag lock (as opposed to the core of a bullet button), one would thread a long set screw through the small upper hole. One reason that the bullet button design took over from these mag locks is that there were huge numbers of gun owners who would unscrew this set screw and use it as an illegal magazine release. The mag lock was designed to allow ready conversion for legal out-of-state use, but actual compliance amongst the owners of these locks was far lower.
Now that the magazine release is installed and functional, it's time to install the B-15 bullet button housing (bottom right of the image).
The B-15 has a small machined recess for the magazine release button to fit into.
The B-15 in place, but not secured yet.
A long, thin bolt with a spring on it. This is actually what keeps the B-15 connected to the receiver.
The bolt goes through the top of the housing, into the top hole on the Prince50 lock. This is what holds the housing to the mag lock.
Screwing the bolt into the B-15.
The bolt, tightened down to the appropriate level.
The B-15 housing, as it appears when properly installed on a receiver.
Using the allen key as a magazine release tool. It's designed to allow a 5.56mm bullet to be used as a tool to detach the magazine.
-- SeanNewton - 03 Jun 2012