Why a resized ring keeps its marks

When we resize a ring, we cut it at the back, opposite the marks (sample note). The maker’s mark, fineness and date letter stay untouched.
We add or remove metal of the same fineness, so an 18ct ring stays 18ct all the way round. The solder is matched to the metal too.
After filing and polishing, you should not be able to find the join. If you can, bring it back.