Could the M476 actually be a M47C ?
In which case it would be a BSA bolt and would not have originally been on a Lithgowicon rifle - however if it went through an armourer they would not sort out 'matching' bolts (so Peter tells us) but it would have had the rifle serial number stamped onto it.

Seems a bit of a conundrum - probably / possibly a 'civilian' job to replace a lost bolt ?

Seems a bit odd that the headspace is too-tight - has it been fired like that ?
Maybe its been used with the notoriously 'thin-rimmed' US made ammunition (Rem, Win, S&B etc)

Do you have any more info re its recent history or usage ?