Volley sights!
Distinct straight sided sight protectors on the Mk.I nosecaps. Also note the piling swivel omitted on post-WW1 SMLEs.
Windage adjustable Mk.I rear sights, calibrated for Mk.VII .303 ammo making it a MK.I***. The Mk.I series had the rear sight protectors on the rear handguard, instead of attached to the barrel as in the Mk.III. The Mk.III handguard and Mk.I barrel on this one leaving it without a protector.