The forend shown in the picture above is for an ejector gun, which would have had a roller latch to start with. If the roller latch is working properly, it should hold regardless of the j-spring. The thumb piece for the splinter FE and the BT FE are different in that the BT thumb piece is longer due to the additional thickness of the wood. If the correct thumb piece is not used the roller latch will not work. I have built up the end of the j-spring by TIG welding a bit of metal on the tip. The j-spring must be clamped between 2 plates of aluminum while welding to preserve its temper.
