RETURN TABLE is converted to RETURN SETOF int in Postgres function
What you see is obviously a display bug in the current pgAdmin 1.20. I could reproduce it.

You must be aware that the code you see is re-engineered from system table entries.

If you look up your function in the system catalogs directly, you'll find that your the return type has been registered properly (at least it works for me in pg 9.4):

SELECT pg_get_functiondef(oid)
FROM   pg_proc
WHERE  proname = 'test';


More useful functions like pg_get_functiondef() in the manual. 

You might want to ask for this on the pgAdmin support list:

http://www.postgresql.org/list/pgadmin-support/

Mail to pgadmin-support@postgresql.org


  How can I force the returned column to have a name of identifier?


Your approach with RETURNS TABLE(identifier int) is the right way. RETURNS SETOF does not take a parameter name, unless you combine it with an OUT parameter. Like so:

CREATE OR REPLACE FUNCTION test(parm1 date
                              , parm2 interval
                              , OUT identifier int)
  RETURNS SETOF integer AS ...