During the 1931 to 1933 years only the single downtube frame was used, except on the TT Replica, which kept the duplex frame.
For 1934 they reverted to the duplex frame for all Flyers, and the Rep was phased out. The ‘new’ duplex frames were of different geometry and slightly shorter, hence the tank fitting problem. These frames can easily be visually identified without resorting to measurements as they dropped taper roller headstock bearings at this point, and so the bearing recesses in the headstock lug are much shallower, and can be seen at a glance. Webb fork type changed to suit these frames, with the top crown having an integral bearing housing. Hope this answers your query.