It's definitely a British MkI, the liner has seen better days, but it is almost 100 years old. I can't help with the insignia though, the black line around the shell is also an interesting addition.
Can you tell us what the colours are on the flash, it's to glarey to tell from the pics.
To my eye it looks blue and orange (even possibly yellow.)
Royal Sussex rgt?
The HS on the rim is for the maker Hadfields Ltd 1916-19