Looks like you have an SMLE buttplate on there. Otherwise looks very authentic and probably original. Perhaps even the forend too. Edit: scratch that, No4 buttplate with top ground down - or an Indian type?
Supports the theory that the "MkI" features such as cocking piece, foresight protector, hinged band etc. went out pretty early in production.
The buttplate is stamped on the inside with an "F", and so is the buttplate trap, is it is highly unlikely to be original lol..