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!