In your case these fields are natural key.
Surrogate keys are keys that have no
“business” meaning and are solely used
to identify a record in the table.
Such keys are either database
generated (example: Identity in SQL
Server, Sequence in Oracle,
Sequence/Identity in DB2 UDB etc.) or
system generated values (like
generated via a table in the schema).
Keys are natural if the attribute it
represents is used for identification
independently of the database schema.
What this basically means is that the
keys are natural if people use them
example: Invoice-Numbers, Tax-Ids, SSN
Surrogate Keys vs Natural Keys for Primary Key
I prefer to add surrogate key to separate business and database model management.
Other question is using clustered and nonclustered index on primary key..
If you table changes (non static table, it has high intensive inserts or update), you will get problem with performance in case of using clustered index on non-monotonic increased key.