I took a look at the head of the case shown in the OP.
I don't think this was Japanese used. The only marking that IS present that looks like anything that the Japanese Navy would also have on their cases would be the '6/14'. In Japanese context, without a preceding 'S' to put it in the Showa era, that could then only mean 'Taisho year 14, month 6', i.e. June 1925. However... I'm pretty confident that:
1) That style of date marking is essentially a 'Japanese copy' of the British style, so in this case likely to be a coincidence (and in reality being as Hazord wrote: June 1914), and I say this especially because...
2) ...it would be a woefully late date for the IJN to have imported a British casing still. From memory, the latest date I recall having seen on a British manufactured case that was imported by Japan (or intended for such) was from 1901. It seems to me that from late 1901 onward Japanese arsenals (mainly/solely?) produced the ammo that the Japanese earlier imported from the UK. I don't have official documentation to back this up, so I'm going here by 'specimens seen'.
On a Japanese used case I WOULD expect some form of Japanese marking, either a small anchor, or some other arsenal and/or acceptance mark, and I would NOT expect a 'Cf' marking on it, nor the broad arrow. As far as my inquiries with collectors specialised in British ammo have lead me to learn, both the 'Cf' and the broad arrow would really only be present on cases that were actually USED by the British (or 'accepted for use') and hence as far as those source have informed me, should NOT be present on cases used by other nations. This matter is open for some debate still though, as elaborated on in my article about early IJN ammo, so please don't shoot the messenger if I'm saying anything that's not completely correct here.
The main trouble I have in being certain about any of this, is that I keep trying to turn over stones in all areas of the world to see if there's some official documentation about such matters as the British exports to Japan, but try as I might, to no avail so far. :/
Hopefully someone can fill that void still some day.
Cheers!
O.-