Houghton Library no longer used the 909 field for reporting purposes. We use the 920 instead. See here for further application of the 920 field.
indicators: undefined, leave blank
...
bytes 7-8 your initials (you may be replacing some existing initials with your own)
example:
H09 909 __ $$k A11C13sw
If you are making a fresh holdings record, then fill the first 3 bytes with fill characters (###).
example:
H09 909 $$k ###C13sw