Number assigned to a publication for which the specified postal service permits the use
of a special mailing class privilege.
Indicators
First - Undefined
# - Undefined
Second - Undefined
# - Undefined
Subfield Codes
$a - Postal registration number (NR)
Numbers are right justified and each unused position contains a zero. The hyphen
that may appear between the third and fourth digits on printed sources is not
carried in the MARC record; it may be generated.