[Herald1360]

Mod during production run? Not seen as significant enough to update literature, perhaps because any contemporary repair would tend to replace unusual parts like MR2 with like for like.

Zinc carbon dry cells didn't sit at 1.5V for long once in use so the filament overrun would have been too short to have much effect on life of the valves. Particularly on a set which would see little if any use on batteries anyway!