Very unlikey the lug was for a light, when shooting over open sights at night you need the light source from behind to illuminate the sights.

40 or more years ago my father and I spent many winter nights "defending" our ewes (sheep) against wild pigs, which eat the lambs as they are born. Dad used a sporterised No 1 mk III* Lee enfield, my job was to hold the light, and if we both got it right it was very effective.