Weird, as all the late 80s Reynolds ATB seat tubes, including Magnum, are 1.0/0.7, giving a 27.2mm internal on a 28.6mm OD tube.
My 86' Pioneer cracked through at the ST/TT joint where it had distorted from the fillet and had been reamed out fully to 27.2, leaving very little thickness of metal. The later frames had the reinforcing sleeve/lug at this joint, allowing the tube to be reamed properly. I think it might depend on which model of reinforcing sleeve was used. Some have the parent tube running fully through, probably running 27.2mm. Others have a straight shoulder inside about 10mm up from the bottom edge, the rest of the internal bore is from the lug not the tube, so it's the outer sleeve/lug that dictates the seatpost diameter, not the tube itself, probably 27.0 or 26.8mm.
Looking at this small selection we have between us, I also suspect Andy built the frames with whatever came to hand...or perhaps sometimes didn't ream the distortion fully out, and sometimes didn't put a sleeve on at all (the-duke's).
All the best,
Pause, take a breath and ask yourself: "What would Rastamouse