Roman Balance / Libra / Trutina questions

[Medicus matt]

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

Not exclusively. Equal arm scales have been found in 'Roman' contexts (the lovely set shown below is in the Petrie Museum and is from mid 4th C AD Egypt and, as such, is possibly down to Grecian influence but I've seen an example from Pompeii that is pretty much the same).
Some combined elements of both forms, having two pans, an equal arm AND a small, moveable weight on one of the arms to make fine adjustments to the weigh at one end.