I have only found one Mark 1*** rifle in 25 years in the USA, All the others have the Irish second serial number
with the ER or CR pre fix and there is the G pre fix too. I understand the Irish ER rifles were mostly original while
the CR pre fix rifles have alot of Mark 3 parts (sights & wood). So if you use the Mark 3 you are alright.

I have an Enfield 1904 Mark 1*** that now has a third serial number, the first two being lined out, it is serial
number G 839 now. A grenade launching rifle converted back to service rifle with nice bore