I've used them in 10 recessed lights (installed with the house in 2003) that didn't have clips. I installed LED retrofit lighting. Installing the clips was relatively easy. I had to open them up a bit to fit the outside of the jar. Pry too hard and they won't be left without screws. Basically, I opened up the closed end of the clamp by inserting and twisting a flat head screwdriver. I tried not to touch the open end of the clamp so it would still hold the glass. I didn't use the screws that came with it because I have clips to hold it without them. Using screws might be better, but there really isn't much room for the drill to create holes.
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