Most older apartment buildings would originally have had a master key system. The main problem is that the developers/landlords don't like to pay for extra mastered locks at construction. These are needed to facilitate lock changes when a tenant moves out. Without extra mastered locks available, non system locks are used. Sure, they use the same key type, and may even be pinned for the master key, but they are not part of the system. Eventually a tenants key will open several other units, because of the master pinning in the locks. Unless you have a maintenance tech who knows how a master key systems works, and can add extra locks to the system, it's going to be a mess.