Question Mark (?) in a circle Armorer Marking (Lithgow)

[Nagloc68]

I spent the best part of a good Friday night researching Lee Enfield Muzzle Erosion gauges and Headspace gauges. I Learned a lot and came across across a thread from 2017 and was advised not to reactivate it, but create a new post. It is rare I can answer a technical/historical question so here I go. Poster asked:

The circle with the question mark in the circle on the reinforce no one on this site has been able to decipher it I cannot find anything on it either.

I had the same mark on a 1915 rifle that was FTR in 1917. And here is the explanation I received from Ian Skennerton, so happy to share and hopefully return the education so many have provided me:
"We figure that the mark is a question without the dot, sometimes seen in a circle as well.
Most likely marked at the factory SAF Lithgow due to the depth of the stamp(s) and finish applied over it, the same as the serial no.
Originally a barrel mark to indicate a difference in rifling or throat features, most likely a relaxation in specifications.
It is not parallel bore, but could be about the throat or maybe depth of rifling.
It came in late in 1916 and we've not seen it on any barrel later than the end of the Great War, which is another reason why we consider it to be a wartime measure.
We have seen it on rifles assembled in 1917 up to the early 1920s and as I mentioned earlier, on some rifles it was stamped on the right side of the action body above or below the serial number.
Obviously where this mark was stamped on the barrel, it could not be seen under the handguard, so in some cases it was stamped on the action body near the serial number."

I attached a link to the original post below: titled "1916 Lithgow SMLE Mk. III"

https://www.google.com/url?client=in...HIffWMINzYKZzW

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