There are some plastic ring compressors used for small engine repair and two of these kits at about $9 each might be worth the investment to make things easier. They are made of red plastic.
Some have used wide worm clamps but taking them apart to pull them out after fitting the pistons into the cylinders are fiddly. The red plastic ones have a finger that projects out from the rings and you wrap a rubber band around that to hold the compression. To remove you pull the rubber band clamping it together and then spread it enough to slip off and out of the way.
At factory, probably did it by hand and skills developed from years of doing it and doing it quickly.
A paint or powder coat would work best on the APE studs... A good high solids paint capable of high heat will provide good protection.
