My early '50s Armstrong Consort has the single boss, and as Neal pointed out, was also originally equipped with a Cyclo derailleur. My 1950 Sun Wasp is the same, just with DB 531 tubing instead of the straight-gauge of the Armstrong. They both ride wonderfully, all these years later, with the Armstrong currently in fixed mode and the Wasp running a 1950 Sturmey FM four-speed medium ratio IGH, just like the bike at your ebay link (which has a '55 dated hub). I can say that the FM is really excellent for spirited riding, though I am careful with it. I will stay far away from that ebay link. This era of British cycling in terms of equipment, popularity in that country, and of course the huge variety of framebuilders makes it one of the best rabbit holes to fall into for the C&V enthusiast.