Mick,
As I mentioned on faceBlab, I have one silver plated collar... however I did not realise the hat badges were done as well.
Can you check to see if those specimens are done in chrome or silver plate? It may not be a meaningful distinction, since either finish is as likely to have been done as the other. Perhaps you can check from the provenance/direct source of the items, wheter both finishes were in use and whether they were worn by Officers as you mentioned and (perhaps) by the Band... or by the BandMaster etc?
Regards
Mark Corcoran
https://charliebravobooks.com/
Co Author of: “Metal Uniform Embellishments of the Australian Army” Post 1953 (the 'QEII' era) Vol 1: Insignia for Corps and Schools Etc and Vol 2: Insignia for Units and Regiments.