how to find DB names in Postgres 7.1?

In Postgres 7.1, ${PGDATA}/base contains directories named by
numbers!!   They used to be database names.   If a user created a db,
can the db admin know what is that db name (without sending any query to
db server)?

I believe there must have some kind of mapping table exist.  Can anyone
tell me where to find it?

Thanx for your help

LM.Liu