At first I thought this was the Mk.2 variant, but putting the image through reverse google search gave me results for the Mk.1 version. So, for the time being (until or unless someone corrects me), the tank will be referred to as a Chieftain Mk.1.

One of the most heavily-armed and armored MBTs of its day, the Chieftain main battle tank was intended primarily as a replacement to both the Centurion and Conqueror; considering it carried the same-caliber gun as the Conqueror while still having relatively decent mobility (in large part due to improved transmission and new powerpack), the Chieftain would've effectively taken up the roles of both predecessor vehicles.

Service-wise, the Chieftain predominantly served in the British Army up until the arrival of the Challenger 1, and was also exported to various Middle Eastern countries (primarily Iran), although most of those would be the later variants like the Mk.3 and 5. The Mk.1 model was, from my understanding, chiefly used by the British and were more or less kept in West Germany to guard the Rhine in the event that the Warsaw Pact countries invaded West Germany proper. Thankfully, this never came to be.

Not sure if the collection had a later model of Chieftain, but I'll cross that bridge once I reach it. I know the information is a lot more brief than what I normally do but I'm trying to pace myself better and not go too off-tangent. Trying to reach that middle ground, pretty much.