Hi Mick, the most obvious way is the rivet holes in the head tube. Gillotts were vertically aligned. Rotrax horizontal...Assuming badges were fitted! I believe before the war taper tube frames from the likes of Selbach and Granby to name but 2 were generally smaller as was the fashion then. I presume Harry Carrington at Gillott thought it would suit a larger frame and aimed and their product at 'The Tall Rider'. So it was described in the catalogue. Harry told me they were still building a few in the mid 50's but had run out of bracket lugs so made that part by fillet brazing. Most people wanted front derailleurs so that was another nail in the taper tubes coffin.
Mark Stevens. Evanton Scotland