Don Firth -

you may well be right with the strop being of that age - but if the original song was older than that, I would expect that the several few centuries of 'folk tradition' intervening would have allowed the 'stropping to sharpen' concept to be seamlessly inserted into older songs - much as 'plug-ins' like the 'milk white steed' formula ... :-)

or the other idea of the 'cutting of the straw' could have been do so too -

Indeed if the 'carriers of the song traditions' were not much acquainted with weapon etiquette, I would expect them to not care much about correctness rather than 'making the story sound good'...