The white pieces are nylon washers.  I use them to space the rail button up just a little bit.  This gives a little more room so that the rail does not rub on the airframe even if the buttons (or the rail) are not perfectly straight.   The nylon is soft enough to conform a little to the roundness of the body tube.

Aft button ready to be installed into the threaded insert that was built into the booster.

Installation complete.  Don't tighten the screw too tight or you will deform the plastic rail button and it won't fit the slot in the rail.  This is why the Loctite is important.  There is very little tension on the screw from the plastic parts and what little there is will change with temperature and age.  A small dab of Loctite 242 thread locker on the screw threads holds the screw in place but still allows it to be removed if necessary.   The Loctite can be found at your local hardware or automotive parts stores.

If there is one flaw with the  Air-X "Black Rock" kit, it is that the motor tube has a true 4.00 inch inside diameter.  This is too large for the standard 98mm motor cases.  In fact, the aft closure on the cases will slide right into the motor tube. Consequently, it is necessary to make a thrust ring that the motor closure can push against.  I also made a retainer ring to hold the motor in place.

These are the thrust plate and the motor retainer ring. They were machined from aluminum and three holes were drilled to allow the three bolts to hold them in place.  The bolts go into the PEM nuts built into the end of the motor tube.

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