This is a Zanzibar Collection from 1964.  Specifically, it is THE REVOLUTION COLLECTION.  What revolution?  We'll get to that.  First, let's talk about this collection and what's in it.

This collection includes the OVERPRINTED Handstamp and Machine 'JAMHURI 1964' Marking, which is SG 394 to 433.  It also includes some ULTRA RARE COVERS with stamps showing the Sultan Crossed out in Ink.  There are 7 Covers like this. 

For more information, see the note in the the Stanley Gibbons Catalogue before #394, including the SG noted overprints on Freedom from Hunger stamps, SG 389.  There's a block of 10 of these on a cover.

It's worth pointing out that some of these stamps were in use for a grand total of 4 DAYS.  You read that correctly, 4 DAYS!  That means that they are VERY RARE, and rarer still to find used on a cover as some of these are.  It's an extremely rare look at a four day window of time during a rather bloody period in the history of Zanzibar.

There is also a separate page with 41 'ERRORS', pieces with DOUBLE OVERPRINTS, pairs with one stamp without the Overprint, inverted overprints, and MORE!

This is a truly engaging and comprehensive collection of these modern revolution issues.  When offered, the errors brought $100 or more, each!

Now, for some backstory and context.  Let's wind the clock back to 1964.  Zanzibar had just become independent from the UK, and the Arab minority succeeded in retaining the hold on power it had inherited from Zanzibar's former existence as an overseas territory of Oman.

Frustrated by under-representation in Parliament, despite winning 54 percent of the vote in the July 1963 election, the African Afro-Shirazi Party (ASP) early in the morning of 12 January 1964, led by John Okello, youth leader of the ASP's Pemba branch, mobilized around 600–800 men on the main island of Unguja (Zanzibar Island). Having overrun the country's police force and appropriated their weaponry, the insurgents proceeded to Zanzibar Town, where they overthrew the Sultan and his government.

The Massacre of Arabs During the Zanzibar Revolution ensued, and the insurgents proceeded to loot Arab and South Asian–owned properties and businesses, and rape or murder Arab and Indian civilians on the island. The death toll is disputed, with estimates ranging from several hundred to 20,000. An estimated 10,000 people escaped from the island, with many fleeing to the UK. The moderate ASP leader Abeid Karume became the country's new president and head of state.

This bit of historic context was taken from Wikipedia.  I urge you to go check it out (link below) and conduct your own further research if it interests you further. 

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, timely manner, and includes tracking!

Citation: https://en.wikipedia.org/wiki/Zanzibar_Revolution#:~:text=The%20Zanzibar%20Revolution%20(Swahili%3A%20Mapinduzi,island's%20majority%20Black%20African%20population.