Sylvia- Slot felts-the white felt pad that the pedal rests on in FLAT position-are there simply to prevent the mechanism from opening up completely, causing a noise or knock. Each chain of the mechanism(C chain, D chain, etc.) can go only so far in the flat AND the sharp direction. If it goes all the way in either direction, you will hear a click or metallic noise. So the slot felt is there to prevent that in the flat direction. But the slot felt, because it is there, will move both the natural and sharp chains in the direction of closed position. If the slot felt is very thick, then in flat position the discs are going to be awfully close to the strings. If the strings are not dead center through the discs, and are instead slightly left of center, as happens to aging harps, then you run a big risk of the string, particularly in the 5th octave, slapping against the lower prong of the sharp disc. So the slot felt only needs to be thick enough to keep the mechanism from opening up completely in the flat direction. I hope this makes sense.
I don’t know. When a harp is brand new, everything works fine. In other words, on a brand new harp, you can use thin slot felts, thick slot felts, or whatever, and they will all do the job. But on an older instrument that is not the case.