From many years of freeing rusted bolts, I found that you don't heat the bolt, you heat the area around it. If you heat the the discs, the same thing happens, they expand in the hole.
I would try penetrating oil, heat disc, and let it cool, hopefully some of the old gunpowder, residue might have softened. Re-do penetrating oil and try to remove. If that fails you might have to have them drilled out.
Like staed they are put in at an angle.
In the picture shown, these do have the "set screws" that hold them in.
I have used Kroil Oil on "frozen" bolts, most of the times it was only for a few minutes and they usually came out, the ones that don't are burned and then drilled smaller and an easy-out used.
L.C. Smith bushings were 5/16"x32 threads and were made from soft steel, British were probably Whitworth.
