It's not a "modified mauser", as posted above - it's a modified "smelly", or SMLE [ short magazine Lee-Enfield], long a standard, in .303, in the British Army before the advent of automatics/self-loaders.
They're easily identifiable by the separate buttstock, joined with a thru-bolt to the receiver socket seen separating the buttstock at/under the bolt handle.
They have a rear locking lug on the bolt, and a replaceable bolt head for headspace adjustments.