Hi, guys!
Can someone tell me, please, how I can replace a default plumbob with something else? And what actually it is - is it a mesh (like sims-heads and clothes) or what? Thanks in advance!

Yes, the plumb-bob is an accessory mesh named xskin-head-arrow-ROOT-ARROW.bmf! It is internally named "head-arrow" because when The Sims was still in development, the "plumb-bob" had the shape of a (rather boring) three-dimensional arrow. You can find the plumb-bob mesh in GameData/Animations/Animations.FAR. You can replace it to any other mesh by moving any homonymous .bmf file name to said Animations folder — even if the mesh in question is the head of a Sim! Keep in mind that its spinning behavior is controlled by an animation file, also in Animations: head-arrow.cmx.

Wow, that's interesting! Going to try it out. Thank you, LUCPIX!

Shaklin has several custom plumbobs here: http://shaklin.chez.com/downloads/objekte02.htm