Pig any age going to slaughter – slapmark, or eartag (needs to be tag which survives scald process, so generally metal), can be tattoo, but most abattoirs hate this so will not take tattoo, and insist on slapmark or tag. Some abattoirs insist on slapmark, so check before marking.
tag is a single tag (doesn't matter which ear), slapmark is twice, once in each shoulder.