Trade and skill badges; metal or cloth?

[cbuehler]

All,
In reference to service dress only, is there any guide line as to the wear of metal or cloth trade and skill at arms badges prior to ww2? I seem to see a mix of both in ww1 and inter-war periods, with perhaps cloth more common between the wars.
Additionally, there seems to be no sure way to date these, although with cloth, I would expect to see more hand embroidery.
Thanks,
CB

[cbuehler]

Any one up for elaborating on this?

CB