A rifle could be DP`d for something as itle as the headspace being on the long side and no parts at hand.
Something else to consider: this is a Mark III rifle. The Mark III went into production at the end of January, 1907. I have a Mark I* converted to ** converted to ***, also 1907, which means the early part of January. At the end of the Great War, huge amounts of equipment was DP`d just because it was older-model stuff: ALL the CLLEs, just for a start. So were many of the Mark I rifle in various mods. I would not be surprised at all to find a very early Mark III which had been DP`d at the same time for no reason other than that it wasn`t the latest model.
Following the Great War, the
British
rebuilt vast numbers of SMLEs. Some were issued in peacetime, others were given to new nations, some were given or sold to the Dominions, others handed out as aid to other countries. Enough were left, completely rebuilt and with brand-new barrels, that when the peaceniks wanted to demonstrate in the 1930s that they were not as warlike as that nice Mister Hitler, thousands of these rifles were DP`d, had the bolts withdrawn, the Bodies crushed and hacksaw cuts put through that brand-new barrel, right at the Knox-form. I know this: I once owned a dozen of them, left another hundred in Lever`s shop.
OTOH, I am shooting a 1907 Mark I*** with prominent DP marking on the barrel. All it needed was a bolt that fit. I have shot this rifle a fair bit over the last 46 years and it hasn`t killed me yet.
I think if the OP`s rifle were examined by someone qualified, likely it could be approved for shooting with mild loads, possibly after minor repairs.... and mild loads will give you your best accuracy, anyway.
Good luck!
A 1907 Mark III. Imagine that! A treasure.
.