Firefly.

This is coolbert:

During the Second World War [WW2], allied [American and British] tank design and development was in a state of ferment the entire period, fielding a range of armor, those vehicles of allied design and manufacture quite often finding themselves in an inferior status - - under gunned in comparison to the German counterpart.

Desperate circumstances require desperate measures. Such a measure was the design and development of the British Firefly tank. An existing American Sherman tank the turret of which was gutted and re-designed  to accommodate the installation of a gun capable of even defeating at range the most potent armor available to the German. And JUST IN TIME FOR THE NORMANDY INVASION!

"The Sherman Firefly was a World War II British variant of the American Sherman tank, fitted with the powerful British 17 pounder anti-tank gun as its main weapon. Originally conceived as a stopgap until future British tank designs came into service"

"It soon became highly valued as the only British tank capable of defeating the Panther and Tiger tanks it faced in Normandy at standard combat ranges"

The Firefly the only tank available to the allies in Normandy [1944] able to kill at longer ranges the very formidable German Panther and Tiger tanks. German Panther and Tiger tanks relatively impervious to existing allied tank and anti-tank guns except at the closest of rangers.

The Firefly an expedient weapons system, existing Sherman tanks having their turrets radically re-engineered to allow for a "fit" of the 17 pounder gun. The Firefly AVAILABLE ONLY TO BRITISH AND DOMINION TROOPS AND NOT AMERICAN!

In the role of tank versus tank, mano-a-mano, the Firefly performing in an excellent manner, for the mission intended more than adequate, a stop-gap measure, wartime modifications done as an expedient to fill a dire need. Given high priority from the command of Churchill himself!

The German tank ace and SS Mann Michael Wittmann and his crew, famous/infamous depending upon your viewpoint, meeting their end on a Normandy battlefield, casualties of an encounter with a Firefly or a massed unit of Firefly, the situation not entirely clear.

coolbert.