For years I've been looking for a supplier of pencil-top erasers that don't leave smudges on sheet music. They are a vital tool for all musicians, who have to grab every free moment to mark up parts in rehearsal. There's no time to fiddle about with separate erasers, and nowhere (except right down there on the floor) to put them safely -- they fall off the stand as you turn the page. A pencil-top eraser serves the dual function of stopping your 4B music pencil from sliding through the hole in the music stand's locking screw AND being always on hand to rub out the bowings left by the previous hirer of the parts, which somehow are always the exact opposite of what the current leader and/or conductor wants.
But could I find any in the shops? Never! The little packs of coloured ones LOOK the part, but in practice they just smear brightly-coloured plasticky stuff all over the precious music. These Faber-Castell ones do actually rub out soft pencil marks, even on nasty old copies that have been almost worn through with repeated re-marking. They are a little on the hard side (softer material would require less pressure and do less damage to delicate paper) but that should at least mean that they last a good while.
Supplier: please correct your entry to spell both "Castell" and "eraser" correctly and make searches easier! Otherwise -- excellent.