Estes Booster 55, Part 2

The "key tabs" on the fins slide into the long slots in the lower plastic side of the tube. They are tight!
When both tabs are in place the fins is slid up until it stops.
NOTE: Be careful not to grip the BT-55 tube at the top too tight, you can bend it fairly easily.

The engine locking ring is spun on until it stops at the base of the fins. The ring also locks the low end of the fins in place. A very well designed accessory.

Here's the top, you can see the inside of the BT-55 tube.

The is a plastic "funnel" that directs the hot propellant pieces into the 18mm engine in the second stage.

Here's the ring that replaces the ring from the RTF kit. You can see the screw threads that fit the upper engine mount.

As John Boren said on a forum, the fit of the "Stager" ring is pretty loose in the Booster 55 unit.
This might be why the few two stage rockets I've flown have veered off when the second stage ignites.