This is a Bahamas OHMS, or ON HIS MAJESTY'S SERVICE Cover, sent from Nassau to Oakland, California on 9/5/1938.

It was sent with a SINGLE RING CROWN CIRCLE 'PAID at BAHAMAS'.  It is likely the latest use of this mark, which by now had been superseded by the double ring mark.  There is also an OVAL TELECOMMUNICATIOS DEPT. NASSAU BAHAMAS in Blue on the lower left.  This is an especially nice and RARE late use.

This item is over $100 USD so I will ship it using Priority Mail in the US, and Registered Mail overseas.  This is to ensure that it reaches you in a safe and timely manner.