Flush Inserts are used is to reinforce the end of a tube. It fits inside and gets glued in the end of the tube to make it stronger; usually used at high stress areas such as the end of the tube that inserted in a ferrel or t-fitting.

The fitting is aluminum... with an inside thread and solid tip. These are also used when running bungi through the tube. With a slight modification, this fitting will hold the knot inside the tube.

SCS40 fits: RET40
OD = .244"