Lewis & Short

lībrātor, ōris, m. [libro].

  1. I. A leveller, esp. by means of a water-level, a surveyor, Cato, R. R. 22, 1; Front. Aquaed. 105: superest ut tu libratorem vel architectum mittas, qui diligenter exploret, sitne lacus altior mari, Plin. Ep. 10, 50, 3.
  2. II. One who throws or hurls weapons by hand (cf. funditor, a slinger): funditores libratoresque excutere tela et proturbare hostem jubet, Tac. A. 2, 20: libratoribus funditoribusque attributus locus, id. ib. 13, 39; Inscr. ap. Kellerm. Vigil. p. 55, n. 127.