Concerning the butt swivel machined out recess, IIRC Peter L mentioned some time ago that some contract woodwork manufactured for foreign governments was produced by modifying existing No4 butts. The oval recess was simply filled with a wood patch. It's possible that the patch may have simply fallen out over time.
Please beware of just shoving a replacement bolt into a LE. It may cycle but unless various parameters have been checked by someone who knows what they are doing, it hasn't been FITTED. It isn't only a question of headspace, but also of proper fit of the locking lugs in the locking lug recesses...........plus correct positioning of the front face of the cocking piece against the sear.........you get the picture. If in doubt I would get the advice of a smith.