This is a Montserrrat First Day Cover, sent in 1935.  Postage was paid for via the KG V Silver Jubilee stamps, including 2 of the 1d, 1 1/2d, and 2 1/2d.  This cover was sent via Registered Mail to California.  It's a real RARITY!

Something to note with this cover is that there are 2 FLAWS on the upper right red 1d stamp.  They include a small red dot to the left of the center flag (Look very carefully at the pictures and you'll see it, but let me know if you can't). 

It also has a large "airplane" flaw in blue/black above the right tower.  Basically, there is a small mark that looks like the silhouette of an airplane or UFO on the stamp.  Thse are not noted in the SG Catalogue!