I think it's a pendulum / plumb-bob.
Check out this pic of a plumb, a square, and a square level from the tomb of Senedjem
at Deir el-Medineh. (Cairo 27280, 27259, 27258)
Ref: http://hbar.phys.msu.ru/gorm/ahist/arnold/arnold.htm
Here's the hieroglyph in question:
--
simon
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