Lee Enfield parts have grown through an evolutionary process along with the Lee Enfield rifles, it can be pretty baffling to know exactly what variation you have or need.
Top left-No1, x, x, Low right-No1Mk3
No1 1907 No1 Mk3 No1 Mk3 Lithgow 1942 Ishapore 1966
Top - No1 circa 1905, note sight blade enclosed by ears plus stacking swivel hole/screw. Bottom - No1 Mk3 1907 1st Variation, note smaller, more open ears plus stacking swivel hole/screw.
Top - 3rd Variation, note opened sight ears for sight adjusting tool plus no stacking swivel hole. Bottom - Ishapore 1966 4th Variation, square sight ears and body cuts plus stacking swivel pillar is removed.