Barrel ring marking 'stupid' question.

I've been trawling through Vol I and both Vol II over the past 4 months of lockdown here in UK, and I've probably missed it staring me in the face, and I've had a search through the forum and not found anything either.....but, what does this marking relate to ringed in red?
This is my 243-1940 which is marked 7,9, and my recently acquired RC 237-1936 is marked 7,91 and a K98k belonging to a fellow shooter which I was looking over at the range last year was marked 7,92. Photos in the Vol I, Vol IIa & b show all 3 as well, but I can't see any mention in the books about this marking?

Sorry if this falls into the blinding obvious category of posts

It's the bore diameter 7.9 millimeter.

It indicates actual bore diameter. IIRC, the minimum allowed was something like 7.89 and the maximum was 7.94.

Bore size. Someone else will tell you why the numbers vary, but 7.9 is 8 mm Mauser.

I had shared in a different thread I can't find now a snip from a book that showed the dimensions and the gauges in a set used to measure it. I probably still have it in some gun folder.

Not the full photo I was looking for but...

Also of note is that the bore marking in that location gets deleted around 1942, so will not be seen on later rifles.

Cheers chaps, every day is a school day as they say

Funny how they decided to do that, mind you they were so anal about stamping everything, it shouldn't be a surprise.

Shame I might not get to see the guy I say last year on the range with his to let him know, as when I looked at his 1939 Erma which has 7,92 and I mentioned "oh yours is 7.92" he obviously thought I didn't know what it was, as he said "Yes, its a 7.92x57 Mauser"
"No, I said, your barrel is marked 7,92, mine is 7,90, here have a look" He didn't realise that when I asked to look at his, I had one as well, and was confused when he saw the 7,90 mark on mine, and I told him I didn't know what it referred to either

I need to find a gunsmith with some gauges to slug my two old girls and see what they are both now, over 80 years later.
The ex-Norwegian Navy 7,90 one is still quite tight with a crisply rifled bore, whereas the RC with 7,91 mark has a much more worn and less shiny bore, although they both shoot about the same in terms of accuracy.

Hi GeeRam, I asked a question about the marking months ago :
https://www.k98kforum.com/showthread.php?44690-Bore-Diameter-Marking