It has a metal but plate with wrap over top lug - the screws look as though they have never been touched and the metal to wood fit has a build up of polish or oil - so it doesn't look like it has been off in the last 100 years - but I may be wrong and certainly worth a look.
I think the brown staining is probably oil residues so it may clean off with soap and water or a mild solvent such as white spirit - I don't want to use anything abrasive as the engraving is very fine and abrasion would dull it - thoughts would be appreciated.
John
