O.H.M.S.

O.H.M.S., an initialism for "On Her Majesty's Service" or "On His Majesty's Service" (depending on the gender of the reigning monarch), is an official franking commonly seen on correspondence from government departments in the United Kingdom, Canada, Australia and other countries that share the same monarch as their head of state (now known as Commonwealth realms).
In Canada, O.H.M.S may be written on an envelope instead of using postage stamps for any letters being sent to the House of Commons of Canada or to any member of the Parliament of Canada. Letters sent from Lieutenant-Governors may also bear the O.H.M.S. mark.
In Australia before Federation, letters in envelopes franked with the O.H.M.S. mark were exempt from postal fees.