Scales for weighing

[M. Demetrius]

Salvete,
I'm interested in making a set of scales for weighing (not squamata for wearing). My question is, what sort would be commonly used for weighing out market-sized portions of things. Like produce, coins, and such as that, not sides of beef.

I know the double pan was used since nearly forever, but what sort of scale would have travelled with a legion, and also be a sort you might find, say in Celtic Britain?

Pictures would always help, since I really do intend to smith one of these. Double pan? Steelyard?

Thanks in advance,
M. Demetrius