This is a British case for the 12 Pounder 12 CWT Naval gun.
Unfortunately, from the dimensions you give, your case appears to have been cut short. This was often done by civilians using cases as ornaments as they thought it improved the look of the case. However: there were various lengths of these types of cases made. It should have three pairs of notches cut around the top of the case at 120 degree angles if it is an original case. These were used to hold a tin cap to keep the powder in place as it was a separately loaded gun.
The “I” at 12 O’Clock means “Mark I case” The /|\ over N is a British Naval acceptace stamp. “LOT. 23” is is the case lot number, “242” could be a loading lot number.
Does this shell case have the notches or not?
Here is a photo of what the top of the case would look like if it was un-cut. However, these notches aren’t strictly original, that case had been cut down by around 4mm to remove the notches. I lathe turned the top of the case straight and re-cut the notches using a milling machine.
Shortened cases are very common here in the UK. After the 1st and 2nd World Wars there were thousands of cases around. Many of these were coverted into ash trays, flower vases, poker stands and other household items. They were often cut down to the length that the user wanted for whatever intended puspose, or to remove dented necks.
I carry a case dimension list and measuring tape to every show and flea market I visit. If the case appears to be cut, I measure and check it agianst the list. If it is cut then I don’t buy it.
You can often tell if a case has been cut. The first clue is saw or file marks across the top of the neck. Also, the neck can look uneven if it has been cut by hand. Case walls that appear unusually thick can also (but not always) be a clue. Many cases are often lathe turned on the inside of the top 1-2 inches of the neck during manufacture. These marks can clearly be seen. These being missing can (but again not always) be a sign that the case is cut.
How long is the original un-cut case?
The original would have been 76.2mm calibre with a 391mm case length.
I’ll post a photo of an un-cut example from my collection tomorrow.
Nice photo, thanks, I think I’ll remember those notches for a long time, I remember visual things better than descriptions.