I would imagine most of Mi5 and Mi6 would have been Freemasons back then, the police certainly were. They were originally set up as a guild for Masons in the 14th century. You could join a guild if you were experienced, skilled craftsman. If you were a member of the guild you would be able to work on big building projects. It all changed in the 17th/18th centuries, you didn’t have to be a Mason, but you had to be invited and sponsored to join. It changed about 20 years ago, now anyone can join!