If I look at the back or the front of mine, there is a solder filled gap between the rib base and the barrel. It looks to be a 32nds wide, or less, but it is there. This is on a BT99 Plus and a Citori Plus Combo. The rib base on the top single for the combo has a rib base that runs the entire length of the barrel and ends at the back just like your BT99 Plus. The BT99 Plus has a small base at the front, about a 1/2" square, a center base and a rather large separate rear base for the back anchor. Mine have very obviously been soldered on the barrel. Hopefully yours are as well. Good luck.
Bob Falfa