I'm looking at the barrel-flat proof marks of a G. O'C. Holloway damascus BLE. They show the marks that would date this gun as having been proofed in Birmingham between 1887-1896. However, there are no marks to indicate whether it was proofed with Schultze or EC nitro loads (as described on pg 163 of 'The Standard Directory of Proof Marks').

Should I assume that this means the gun was black-powder proofed only?
Also, does the 12 over C in the diamond cartouche indicate a chamber length longer than 2.5 inches?

Thanks for any info.

If there is no "NITRO PROOF" marked then its B.P. only .
12 over C denotes 2.5" chamber

Thank you - do you know what a 12 without a C in a diamond cartouche means?

O a Birmingham proofed gun of that era there should be a gauge mark for the more of the gun, no diamond or other enclosure & then the Chamber mark. This will be either 12C or 12LC (chamber & long chamber) It is debatable whether the LC includes 3" chambers or only picks up on chambers longer than 3". It seems 3" chambers have been found marked each way. If it does not have the Nitro mark then as stated it is black powder proof only.

For a number of years, British guns with smokeless proof were actually in fact proofed with a special Black Powder proof powder. This powder gave a high enough pressure to make the proof suitable for smokeless.

I am not familiar with a British proof mark of the gauge in a diamond without either the C or LC

Possibly someone smarter re: British proof marks could clarify, and thank you in advance.

Under the 1896 Rules of Proof, the maker could request Supplementary Nitro Proof with either "E.C." or "Schultze". I don't know when that was permitted prior to 1896
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA2&dq

Standard powders used for proof were at the discretion of the Proof House, and were given as T.P. (Tower Proof) “which shall be of strength equal to Waltham Abbey “R.F.G. 2” and of grain varying between Nos. 4 and 5, that known as Curtis and Harvey's “T.S.” (Treble Strong) No.2 [for Supplementary Nitro Proof with 4 1/2 Drams and 1 2/3 oz. “soft” No. 6 shot] and, that known as “Col. Hawkers Duck Gun Powder”, and the nitro-powder known as Cordite, or any other description of nitro-powder which may hereafter, from time to time, be adopted by His Majesty's War Dept.

I have not seen a British proof mark indicating proof with "E.C." or "Schultze" as was used by the Belgian proof house, nor is one shown here
https://books.google.com/books?id=lvBk8df8PjUC&pg=PA139&lpg



Or this mark indicating proof with E.C. No. 3, a 33 gr. = 3 Dr. Eq. powder



1896 - 1904 some British guns did have the Service Load marked, like this 1896 Lancaster.
42 grains of the original "E.C." Bulk Smokeless was 3 Drams, with 1 1/8 oz. shot



or the MAX. shot charge, but without the grains Nitro powder

But from https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA296&dq

Note the pressures expressed by psi would be by crushers and modern transducer pressures would be 10 - 14% greater




Has anyone seen an E.C. or SCH mark?

Clarification: Do you mean just 12 all by itself, or do you mean 12 in a diamond but no C? If the former, that's simply the bore designation. Means it's a true 12, bore diameter .729. Quite a few British 12's were either somewhat overbored or underbored (much more often the latter, from what I've seen) and you'll often see them marked with bores other than true 12. If marked with a 13/1, for example, that would mean a bore diameter of .719.

I can't recall ever having seen 12 in a diamond but without the C.

Larry, the C in the diamond was discontinued in the changes made in 1954.

The gun in question though is said to have 1887-1896 proof marks. The 1954 rules would not apply except in case of a re-proof & no mention was made of this. The plain 12 without the diamond or C as stated indicates bore diameter, but not necessarily exactly 0.729". What it meant was it would accept a 0.729" plug gage to the required depth of 9" from the breech but would not accept the 12/1 gage of 0.740" diameter, could have been anywhere between those sizes.

Bore dimensions and marks post-1887 Rules of Proof

Here's the site where you can see the gun and proof marks (zoom in on lower right photo):

https://ellwoodepps.com/g-o-c-holloway-co-side-by-side-ejector.html

Also, note the pins (screws) for intercepting sears on the upper rear action sides - unusual for a boxlock I think.

I appreciate everyone's comments and observations, thank you!

It certainly appears you were correct in identifying the proofs as 1887-1896 marks Both the 12/C in diamond & the word Choke rather than not for ball i9ndicate that. Right bore was proofed @ 13 (0.710"<0.719") & left bore @ 13/1 (0.719"<0.729"). Realize as I stated earlier these are not exact dimensions but the "Range" they fall in. It would be quite conceivable that the right bore was 0.718" & the left bore 0.720" which would be compatible with the marks but in fact, there would only be 0.002" difference between the two bores.