I think just being in a holster doesn't wear the "etching" off of them as much as the removing/re-holstering of them many times over years does. I imagine vigorous cleaning might do the same thing.
The middle 4513 I've had for years and it's been taken out and put back in different holsters many times. The "etching" is definitely lighter than the "new", (top,) 4013 is. The lower 4513 spent many hours in a holster, (judging by the worn right side of the grip and rear sight,) but is still fairly dark.
My CS's "etchings" are getting fairly worn too.
Jim
Jim