This is a British Honduras OHMS, or ON HIS/HER MAJESTY'S SERVICE Registered Cover.  This one was sent from Belize to St. Joseph, MI via New Orleans as per the backstamp.

What makes this little beauty really special is the fact that it was sent with a 25 Cents British Honduras SG 157 stamp to pay the Registration Fee.  It also has US Censor tape and the RARE PASSED BY EXAMINER HANDSTAMP.