I was looking up both RH Clark and RH Clarke (later when I also thought it could be a mistake), nothing showed up for RH Clark for me, however I did find one thing while searching in google books. It was some book all about Singapore, and there is a list of Officers in different units, and when it underlines RH Clarke, No 80. Company, Royal Garrison Artillery stationed in Singapore. I believe Singapore was a British colony and or trading post? However this tunic being later Victorian may dismiss anyone past 1902 but not impossible, i'm not sure if officers were allowed to wear the pre-1902 pattern of Royal Artillery tunics if they had already had one when they were commissioned before the reforms. Thank you for your assistance.
Being c.1912 it may still be possible for it to be Pre-ww1 Georgian however 2 years is a very slim amount of time. And it could have existed earlier and so far no trace of it, as you said. I believe the Rifle Volunteer movement was popular from about the 1860s-WW1, but we are still not positive if this is even a Rifle Volunteer Pill Box. This is a very hard object to uncover it's past, but I kind of like this sort of "treasure hunt" for information. Thank you for your assistance.
I have been looking for similar things on google images and pinterest and such however nothing really has shown up so far. I have seen one similar but it was still quite different in a few ways and not the same cap. And it was Victorian like 1880s or something. Thank you for your help and encouragement, though.