6.5 Arisaka
I had a look at the reference you gave for the cartridge collector's exchange and he should be ashamed of himself! Grenade blanks my...***.!
As it happens, you have come to the right place, because I wrote the best (because it is the only) book on the Arisaka in British service. We had 150,000 Type 30 and 38 rifles and carbines and they were known as .256" calibre in British service.
The first 50,000 went to the Royal Navy to release Lee Enfields for the army at the end of 1914 and the remainder were used to train the Kitchener New Armies. There was no "Home Guard" in WWI, although there was a volunteer defence corps, but they had Martinis.
As the production of L-E rifles increased, so the need for the non standard Arisakas lessened. Japan had also supplied Russia with about 400,000 Arisakas so we sent most of ours to Russia starting in 1916, eventually sending about 128,000. We sent a few to the Arabs under Lawrence and had about 17,000 left in stock at the end of the war. These were eventually sold and ended up in Finland.
Initially we used Japanese ammo with the Type 38 spitzer bullet which was the .256 inch Mark I, then Kynoch and Royal Laboratory started manufacturing the older Type 30 round nosed bullet. This was mainly for use by Russia but was also used by UK troops and was the .256 inch Mark II. It is one of these that is shown on that web site. The Kynoch rounds were headstamped in 1916 and 1917, but the RL production had a plain head. It can be fairly easily identified though because it uses a small copper primer that is invariably slightly off centre.
That is a very brief synopsis of the British connection but the book tells all, including the rifles and carbines, ammo, drill rounds, chargers and packaging etc., etc.
End of commercial break!
Cheers
TonyE