Roman Balance / Libra / Trutina questions

[D B Campbell]

... an old iron two-pan balance.

The Romans (as far as I know) used the steelyard, rather than the (Greek) two-pan balance.

Using a steelyard requires only a single counterweight, which moves along the bar to achieve the balance (rather than adding more weight to a pan). The counterweights are usually highly decorative, often in the form of a bust.
Example from Daremberg-Saglio.