https://www.facebook.com/photo.php?fbid=...eater&ifg=1

To "judge" the double in an 100% correct way, I need better pictures, included the stamps.

Heym build severall doubles (boxlock and sidelocks) for India in ca. 1915 - 1940.
There are some, I have some in my archive.

And:
Please make sure, that you use only the correct amo for that rifle.
.375 HH vs. the flanged Version is not the same.
Maybe you can fire them both, maybe it "works" one or two times, but it can be very dangerous.
Please dont do it.