The Panther didn't appear on the battlefield until Kursk on the Eastern Front in July 1943 after which Britain and the US learned about them. But it was early 1944 before the British and the US armies encountered them at Anzio.Interesting how different designs prevailed for longer. The German change to sloped armour came about due to
contact with opposition vehicles whereas the British didn't encounter it in enemy vehicles for the first half of the war.