David is correct

>>> dbm.open.__doc__
"Open or create database at path given by *file*.\n\n    Optional argument *flag* can be 'r' (default) for read-only access, 'w'\n    for read-write access of an existing database, 'c' for read-write access\n    to a new or existing database, and 'n' for read-write access to a new\n    database.\n\n    Note: 'r' and 'w' fail if the database doesn't exist; 'c' creates it\n    only if it doesn't exist; and 'n' always creates a new database.\n    "
>>> help(dbm.open)
Help on function open in module dbm:

open(file, flag='r', mode=438)
    Open or create database at path given by *file*.
...    

IDLE tooltip (still using inspect.getfullargspec) also shows
open(file, flag='r', mode=438)
The int comes from dbm.open.__defaults__[1]
438