BSA did originally advertise the Mercury as a cross between the sporting lines of the Airsporter and break barrel action of the Meteor, so I guess it is not beyond the realms of possibility that a Meteor barrel could have ended up on a Mercury. Or some later owner could have replaced it. No idea about the SN I'm afraid. It may be worth asking BSA or perhaps an ex employee of the company reads this thread and can answer your very interesting question.
Kind regards,
John