Leon,
Well spotted. Indeed the first few DTs did have a number of vestigial RA features. Such as you pointed out; the leaf spring saddle and lugs for the RA brakes fore and aft. Also the petrol tank has the petrol tap wells from the RA as well as the provision to hold two spare spark plugs at the nose of the petrol tank. There is an example of just such an early DT presently residing in Tasmania. This has a TF frame prefix, so it is a DT, where as the RA28 has an SF prefix.
According to Jeff Clew’s book the RA28 used an engine prefix of EK, immediately preceding the EL prefix used on the DT engine (just like the frame prefix). One such engine is known to exist, and it is definitely an RA motor in all but prefix. If it were not for that, the RA28 could have been a very early DT, as illustrated, with a DT motor and with the surplus chassis lugs. While the RA28 used an RA motor (more left-overs, no doubt), it is clear from the frame they did not intend to use the RA gearbox mounted to the engine. Many of the DT frame components at the forward end, including the entire headstock, are left-over RA.
So far, no pictures of an early DT have come to light showing one fitted with an RA motor. If one did, it could be an RA28 and I am sure Howard would be most interested to see it. There were certainly plenty of second-hand RA motors placed into DTs when the original DT engine was damaged beyond repair. Usually these have the then no longer required gear box mounting platform and sometimes hand change anchorage cut away from the crankcase casting. These did not have a very long life on the cinders, as they were not as robust as the later, improved DT. So it would have to be an RA motor in an early DT frame to be a contender for an RA28. Except for the engine, there seemed to be very little separating the RA28 from the DT. It is not known if they even bothered to fit any of the brake components to the RA28, minimizing the differences even further. The brake anchorage lugs may have already been assembled on the inventory of tubes that the factory was using up, hence having them still on the first batch of DT models manufactured.
-Doug