I have also found what the X view marks meant.

Apparently my Snider was built by the London Small Arms company. Again, apparently not a lot were made by them so thats kind of neat. The lock is a Enfield lock though. So must have been replaced at some point. I cant see a date marking on the barrel, though after cleaning there is pitting but not as much as I have on my other rifles.

I have also cleaned up under the breech and the marking are more clear now. I think the serial number is either 0290 or O290. Also got M16 markings as well?

https://imgur.com/gallery/HNbomTN