It looks like this Part has been superseded by LEGO Part 56823 String Cord Thin [Undetermined Length].
Click on a colored part below to see the Sets and MOCs that use part rb00166 String Cord Thick in that color. You can find an overview of all LEGO Colors here.
If a color is missing, it probably needs to be added to a Set Inventory.
There are 4 Mold variations of part rb00166 - String Cord Thick in the database, in use between 1973 to 2003. See Molds and other related Parts.