“139. Offence of having article with blade or point in public place. (1) Subject to subsections (4) and (5) below, any person who has an article to which this section applies with him in a public place shall be guilty of an offence….. (4) It shall be a defence for a person charged with an offence under this section to prove that he had good reason or lawful authority for having the article with him in a public place.
“In deciding whether a reasonable excuse is made out for the carrying of an offensive weapon in a public place the court should ask whether a reasonable man would accept that in the particular circumstances it was a proper occasion for carrying such a weapon.”