Once there was more than one factory, and especially when the shooting started and every man and his dog was making parts, the chances of a "PURE" ANYTHING went right out the window.
It doesn't matter to an Ordnance / workshop system. If the parts are as per the current instructions and have met gauging and metallurgical testing, and the whole assembly passes function and firing tests, it is all Kosher.
OK, if you are lucky enough to find a Pre-WW1 Lithgow, still in its Mk 111 guise and with ALL Lithgow parts, it is a rarity of a significant order. Or, it may be a total bodge job. Once such a rifle entered service in Oz, it would have been bashed around the parade grounds and battlefields for the rest of its life.
Consider the number of SMLEs that went from Australiato Britain
in WW1 (and 2). Quite a few were officially repaired, rebuilt, upgraded etc. during that fracas. If such an "officially, by the book" rifle was then further upgraded to "111*" status, it is still "authentic", especially if appropriately marked. Some of the Lithgow rifles returned to Australia, in the hands of Austrailan troops, who were issued or "acquired" them in North Africa.
If the Unit or Field Workshop armourers declared any of these "tourist" rifles to be in need of substantial repair work, they went to big "Base" workshops for a total rebuild.
They even swapped actions around to get "product" out the door.
Back to a Unit for a bit more thrashing.
Sure, many of us would like to have a mint, untouched 1912-marked Lithgow (with matching bayonet, of course), BUT these are SERVICE rifles. Just like SERVICE vehicles and even aircraft, they were used, abused and rebuilt. How many Lancaster bombers survived WW2 with their original engines, tyres and guns, let alone crews?
If you change the tracks and drive sprockets on your "mint' M-4 tank, is it any less authentic?
With Commonwealth gear, every component of any significance (even the wooden ones) were numbered.
If you are into US gear, where do you start?