Usually the conversion was done to allow the retention of the firearm following the UK's legislative "reforms" of 1968. Shotguns could be kept on a certificate, however rifles required a licence as I understand it. The issuing of licences was stringently controlled and many firearm owners avoided the trouble by having their rook rifles smooth-bored. Sadly, many paradoxes had their rifled chokes bored out at that time also.

I have a similar firearm in my collection, a .450 Rook converted to 28-bore shotgun, with expertly removed rear sight block. Possibly made by John Thomas, as it bears his triangle patent mark that appears on the flats of many (most?) of Holland & Holland's top-lever hammerless rook rifles, perhaps even the one pictured above!