very good advice from Raymond, as usual.
some old locks have d black patina if thats the right word for blackening probably from age and some alloy in the brass (brass often contains lead as well as the zinc and copper to make it machine easily,
this makes seeing a code number difficult.
One trick that will show a code number that is stamped in however lightly and obscured by blackening of the metal is to use a white powder on the surface, anything from baby talcum or even flour baking soda foot powder, it just has to be finely ground and white.
dust this on lightly and wipe the surface, this will leave only traces of the powder in the low spots where the code stamp is, and should make it easy to read.
Wake up and smell the Kafka!!!