This is a Leeward Islands OHMS, or ON HIS/HER MAJESTY'S SERVICE Cover.  This one was sent from Dominica to Hull, on 12/7/1918 via Registered Mail.  It was forwarded to Harrogate.

What makes this little beauty really special is the fact that it was registered with a some unusual First Issue stamps, the Leeward 1/2d to the 2 1/2d.  These are very late usages.  It is also backstamped "In Transit" on the back

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!