Oh, well in that case.....I defer to Russ' expertise.

That said, there WAS a .500/.450 3 1/4 NITRO Velopex 365 grain bullet loading that may well have been used to beat it into needing a rejoint.
Said 365 grain load would have been just as stable as your noted 350 grain.....not to say regulation would have been great, but maybe it was acceptable to whomever was using it?
IF reregulated for Nitro 365's; then likely not reregulated in England, it would have to have gone thru reproof and be so marked.
- Mike