On re-reading this thread (Wee pen knife) from six years ago -- which has just been revived -- I found something which relates to this thread:

The link that Malcolm Douglas provided in his post, after discussing weapon knife vs. wee pen knife, goes on to mention:
"...please note the Hugh the Graeme error in MacColl or the Corries songbook, or both - his sword was not of the 'the metal bent', or 'the metal burnt', but of 'the metal broon' in contrast to his other sword which is of the 'metal clear' "

Not having a phenomenal memory, I did not happen to remember the factoid contained in this link all these years later, and of course -- sadly -- Malcolm was no longer around to comment... but his it's brilliant that his vast knowledge is still here to provide enlightenment -- albeit sometimes in unexpected ways...

Although this does not clarify things completely (the words bent and brown are both used in the line(s) in question, it was a buzz to re-read that old thread (and link) and make a new connection...