The Forster 3-way cutter slides over the Forster shaft without play. That Forster shaft measures .490" in diameter. A measurment of your Redding shaft diameter is the first thing you need to know.
I'm sure you are aware that on a Forster Trimmer, the cutter rotates while the case is collated and stationary. The Redding is opposite that. The cutter is stationary. Be aware that it is possible that the direction of the cutting head could mean that you have to rotate the handle the opposite way, even if it does fit.