One thing to keep in mind is replacement parts and warranty issues. Align parts are easy to come by. Not sure about anything sold on eBay.
I have 2 Align power feeds on my mill. After 5 years no issues.
The unit actually installed fine (so far). It has the two screws that fit in slots and this is adjusted in and out along the X axis to mesh the gears. The gear fit right on the end of the leadscrew. The brackets also fit pretty well. I didn't crank them down too tight since I saw another post that said to be careful or the brackets will crack. I may make something to attach this better.I have installed 3 of these so far, an Align, a Bestline and something that wasn’t either of those. Two were X axis, one was Z axis.
From direct experience, I can tell you these is not an iota of difference in these units. The instructions are equally deplorable and basically useless. Throw them away and start figuring it out. The main issue that comes up is how many shims to use and how would you know. I’ve tried analytic measurement and trial and error, lots of error. Neither is particularly satisfactory, especially because the criteria for success is not clear. Good luck, eventually you will arrive at a compromise between smoothness and what you hope is minimum wear.
If you are replacing a Bridgeport branded power feed, be aware that, at least in my case, there was a shaft extension required that was something like $120...when it came, I could have made it easily for zilch.
So I suggest getting the higher torque model at the lowest cost possible.
Those already have springs. But, I figured it out. They go inside the black plastic cover over the plungers on the plunger. They hold the cover on the plunger unit. Here's a view looking at the plunger assembly from the bottom. You can see the spring.The springs mentioned are, I believe, for the end travel stops. They fit on the plunger looking thing that contacts the switch and provide some cushioning, since the transition to stopped is not instantaneous.
That does not sound like the units I installed.It has the two screws that fit in slots and this is adjusted in and out along the X axis to mesh the gears.
I wouldn't think a unit designed for a BP type mill would work on a RF30, it's not just the mounting but also the interface to the lead screw.