A 26.8 seatpin would seem to imply a seattube (assuming a standard Imperial 11/8" OD seattube) with a wall thickness of .8mm at the seat end. I'm arriving at that as follows: 28.6 mm OD (metric equivalent of 11/8") minus 2 times .8mm wall thickness equals 27.0 mm. Subtract .2mm for insertion clearance, for 26.8 mm seatpin size. However, British tubing manufacturers measure wall thickness in Standard Wire Guage (SWG) so actual thickness may be slightly different in conversion to metric. Might a frame of this vintage be built of Accles and Pollack tubing, as well as Reynolds? Also, didn't Raleigh Internationals use a 26.8 seatpin? Would this be the SWG equivalent of a 1.1/.8 mm butted seattube?