Howard,
I know you have turned up some pictures of real oddity frames, but I still think this one looks like a typical DT! One last check would be to see if the headstock has taper roller bearings (TT/I.o.M., and OC) instead of balls (DT/SW). The head stock lugs do not look large enough for the taper roller races (but it is hard to be sure), and the tube angle on the upper lug is all wrong for a TT but correct for a DT/SW.
One can not even say it is an early TT frame, as there are several period photos published of 1925 TT machines, and they only visually differed from the 1926-28 models in that they still had the earlier rear axle forks like the RA and OB, predating the cast lug as used on later models like the OC, DT/SW and of course 1926-28 TT/I.o.M. An excellent example (except for the nickel plating) of a early TT is the late Rex Judd machine on display at the Whitewebb Museum of Transport, Enfield, UK.
If the frame prefix is legit, then I am at a loss to explain! Make sure the owner is reading the number off the left-hand side of the gearbox mounting lug, and not some 'new number' stamped elsewhere!
The gearbox is not a problem. The only difference between the DT and the TT/I.o.M. ‘box is the prefix stamped on the case, and the ratios inside. Otherwise they are interchangeable. In fact my SW has a UG prefix box fitted, but it still had the DT ratios inside from a 1950s restoration. There are short and then improved long taper input sleeve gears, but so long as you use the corresponding sprocket hub and proper chain offset, all will be well.
By the by, most of the TT/I.o.M. models in the LDMCC register, have replacment EL prefix DT/SW engines!
-Doug