Looks like a Cyril Adams gun. I have a friend that bought a 28 gauge hammer gun that had been sleeved down from a 20 gauge.
Remember he once owned Atkin Grant & Lang so he had the right people working for him that could do anything possible.
My friend also owned one of his guns as well, a Robertson 20 bore that Cyril had sleeved out to 30" to shoot doves in South America with.


Mike Proctor