Right a few corrections:-
If it's 303 and that seems highly likely it's not a Martini Henry as those are 577/450. It's either a MM (Martini Metford) ot ME (martini Enfield) however that one is not British!
Not I don't think it's a native knock off either, it's an Omani carbine more commonly know as a Muscat Martini. These were made in England or Belguim for the Sultan of Oman and it seems Muscat is the capital. Here is the breech marking of mine:-
see the similarities? I can even make out bits of the stippling down the top of the barrel. On the barrel should be proof markings, these are Belgian ones :-
Mine has some native repairs inside but it functions well even if the chambers shoulder is not quite to British 303 specs.
Oh it's assembled wrong the cocking indicator should point upwards when un-cocked and backwards when cocked. There is a museum in Muscat at one of the old forts and it has some of these on show. Hope that helps