The BT-55 section will slide over the lower BT-50 body tube from the rear.
You don't want the centering rings to catch and roll over the edges of the BT-50 body tube.
Use a smooth dowel or the back end of a Sharpie pen to roll over and smooth out the rear edge of the BT-50 body tube.
The inside edges of the 50/55 centering rings should be sanded to allow an easier sliding fit over the lower BT-50 body tube.
Take a good look at the inside edges and remove any rolled over edges with 220 grit sandpaper.
I marked the top (T) and bottom (B) of the BT-55 section for reference.
The top end of the BT-55 section was already slipped over the end of the BT-50 tube lower end.
This picture was taken to show an area of concern. This area takes an extra moment to line up the inside BT-50 tube through the rear centering ring.
You may have to sand away some of the centering ring lip forward of the letter "B".
I noticed a crack forming where the filler was in the centering ring seam.
The BT-55 section was removed.
To fill the ring a bead of medium CA was set around the ring and sanded smooth.