I can't add to the 'gen' on the song but, in Oz, the word "trouser" is not used as a (singular) noun as the name for the garment, but as a verb; "to trouser" something (usually money or valuables) is "to pocket" it, often with the allusion that the action is illegal or disreputable.

And, while the Oz slang "strides" and "duds" are both still used (as per Helen's post of almost exactly 10 years ago), older males often use the word "daks". I gather (from the Recitations anyone? thread, which includes the text of the Ballad of Idwal Slabs)) that "Daks" was a brand of trews made in the UK.

Cheers, Rowan