Letter Trade Badges - What do they mean?

[Shiny]

Hi All,

I've got my hands on a bag of badges recently, one of which was the A in a wreath as per the attached photo.

I know it is a trade badge and I know there is a series of them with different letters but is there a list anywhere of what they all are - as in what would you have to do to qualify to wear an A trade badge or the B trade and so on?

What classes as an A trade?

Thanks a lot,

Michael