You picked up a very niffy piece of kit there Buckstix!
DoubleD is the man to get these old ladies singing sweetly again & he might be able to assist with the cyphers on your rifle.

I've added in this from a MH site which shows the markings on military rifles - this obviously isn't but hopefully it may help?
http://www.martinihenry.org/index.php?route=product/category&path=61_66

The adaptor cartridge is a nifty piece too, just be careful when first using as they can stick due to how some of the chambers were cut - Douglas can explain a lot better than me!
I think he used a stiff bronze brush on a flexible rod to grab it should it do so.

I would still do the bore test to determine the grove diameter regardless
Still too busy to get out & give mine a lap, one day I'll get there!