AL-310S is just the p/n for the power feed drive/motor assy. It doesn't denote which mill it is for, it's used with various kits.
Did you buy a power feed for a knee mill or for a mill/drill? If you have a brass gear with 5/8" bore it sounds like you bought one for a knee mill.
If you buy one for a mill/drill it will come with everything you need to bolt onto any of the Rong Fu mill/drills. It will come with a spline gear that takes the place of the stock handwheel & is driven by the spline, held on with a set screw, no key needed. And from what I have seen the gear is always steel & comes with a 17mm bore.
If your power feed came with a bracket & gear that looks like this, it's for a knee mill. You can still use it but you will have to modify parts to adapt it to work with a mill/drill.
Power feeds for mill/drill, like the one you need for your RF-31, will look similar to this. Note the mounting bracket & gear. Some will relocate the rapid switch to the mounting bracket, some will not.
I've purchase powerfeed replacement parts from
Bestline Products before but they don't list the spline gear I was referring to on their site. They might be able to supply it for you if you give them a call but it may not help you much as it's designed to be used with the power feed drive mounted horizontally & you'll need the mill/drill bracket for that. If the brass gear pictured above is what you have, you'll need to mount the power feed drive vertically.