Maybe we should say that prior to 1925 the 12 over C in a diamond meant chamber length of 3" or longer.

This is a 16g with pre-1925 proof with 2 1/2" chambers

https://photos.smugmug.com/Proof-Mar...925Proof-M.jpg

Engelhardt in the 1962 Gun Digest "European Proof Marks, Proof in Great Britain" stated that the 12 over C was not used prior to 1925, which is obviously incorrect, and didn't mention 12 over LC.

I think we can say that gauge over C means "standard chamber length" - 2 1/2" to 3" (after 1925 anyway) and 'LC' means 'long chamber"

With this circa 1900 Greener, maybe the 12 over C in a diamond actually means a chamber length over 2 1/2". We see that mark in guns with 3 1/4" chambers prior to 1925. This 12 over C mark is for 2 3/4" chambers prior to 1925.
https://i.imgur.com/uYEutY0.jpg

a 1925 or later example. Lots of questions , but luckily we have lots of examples.

https://i.imgur.com/a4f9hbb.jpg

Can we now say that 12 over C in the diamond, prior to 1925 means a 12 ga chamber that could be anything from 2 1/2" to at least 3 1/4" [probably more] . Then after 1925 the 12 over C in the diamond would be proper for chambers up to, but including , chambers of 3". Then the 12 over LC would be used for longer than 3" and would also be marked with numerical length ?

A later thought is that the gauge over C in the diamond just meant a gun with chambers [any length], Earlier there were centerfires and of course muzzle loaders that did not have chambers, thus the C designation under the gauge .

Thanks Daryl.
The author didn't try to address the C here :(
https://www.vintageguns.co.uk/magazi...roof-6-london-

Found the 1904 Proof House Revisions in The Gun and Its Development, 1907
http://books.google.com/books?id=3HMCAAAAYAAJ&pg=PA290
No specific mention of the 12 over C chamber lengths
LC is for chambers "more than 3 inches"

https://photos.smugmug.com/Proof-Mar...t%201907-M.png

So the 1925 date is incorrect

Good find , Drew, in the 1904 revisions. Longer than 3” seems right for LC. Maybe up until that time. LC did not exist and gauge over C in the diamond could be any chamber length. I think we are making progress.

The 1896 Rules with both C and LC
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA4

https://photos.smugmug.com/Proof-Mar...%20Rules-L.png

Rule #22 if for 3" or longer chambers

Rule #40 specifies the 'C' but does not mention chamber length

SO it appears that the Rule was changed for LC in 1904

Drew, how did you decide that the rule for LC was changed in 1904. It seems that the LC you show above is part of the 1896 Rules.

This states under the 1887 - 1896 Rules LC is 3" or longer
https://www.vintageguns.co.uk/magazi...roof-4-london-
I couldn't however find the Rules published

96 - 04 Rules 3" or longer
Rule # 45 refers to Rule #22
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA3#

04' Rules state LC for is more than 3"