Many thanks for all your help guys. I raided my favourite second-hand tool shop for sockets (as suggested) and this is the result:
Measured using a Britool extension and a 9/16” Fuller 3/8 drive socket, wrapped in increasing single layers of paint masking tape, the bores at 9” from the beech end are both c. 0.845”. Both chokes were between (go) 0.797” and (no-go) 0.803”, (1 more wrap of tape). So, I wouldn't think it has been reamed as the proof mark of 8/1 corresponds to 0.847" minimum, and these bores are slightly tighter.
46 points is the nominal Full choke in this bore. These are both between 42 and 48 points.
Someone recently told me collectors are pack rats. By happy coincidence I came across an 8-bore cleaning brush I purchased from Napier products several years ago and had since entirely forgotten. These are very well made in all bore sizes, (including 8 and 10 bore). I also paid out a while back for an ebony coloured cleaning rod of Parker-Hale style that was somewhat longer than most. For the 32" barrels of this shotgun it and the aforementioned bore brush are perfect companions.
