Before you try a new bolt body, go through a heap of boltheads and see if there's a new or used 2 or 3 that will bring it up to CHS spec. We learned that the actual bolt head number didn't mean a lot - a bit like L1A1 or Bren locking shoulder numbers. The one that gives you the CHS is the one that you need, regardless of the size stamped on it. But if there's two that bring it into spec, then obviously fit the smaller one. But avoid a No4 size like the plague because this was NEVER a UKMilitary size nor was a No4 size made at the UK factories (..................oh no it wasn't..............).