I would think 35/1572 is simply a later replacement frame supplied by the factory. The HPN prefixes are speculated to also be replacement frames, but that does not mean all replacement frames had that prefix, or when they might have initiated (or stopped) that practice. The fact that the character fonts match the typical Douglas serial number stamps I think lends credence this was a Mark 1 returned to the factory for a legitimate frame replacement, and the factory stamped it with its original number. I have no idea if there was a legal requirement to note this in the old log books, or if anyone bothered to comply if there was!
So why did they not stamp the full prefix? Who knows. Perhaps they had worn out or lost the 'T' and 'S' stamp. Perhaps they did not feel it necessary; the numbers being the key element of unique identification. Also too, they eventually stopped the practice of stamping ANY prefix on the regular production numbering at some point during the Mark 3 run.
The frame does have the features of a later frame. Besides the anchorage for the rear subframe down low on the gusset as pointed out, it also has the rear frame lug terminating square to the vertical tube, rather than scarfed on an angle like the (early) Mark 3. The headstock lug is also the heavy, square lump at the petrol tank mounting rather than the slim round boss seen on the earlier bikes.