Screwplates

[Koopyetz]

Hello:
I would like to know if there is a type classification for the threaded screwplates that affix the Order to the uniform. Are there different metals used in the manufacturer of the plates. I have noticed that some screwplates have two small roundish raised portions of metal ( one on each side of threaded opening). What might you call these and is the function to aid in tightening the plate? I also notice that one screwplate may be of a different thickness to another screwplate. Can one tell by looking at a sample determine if it is from an earlier Order or a more recent piece.
Thank you in advance for your help.
Regards
Rick