Purchase Date being April 1978 + No Warranty + 5G model number

Hello, a week ago I bought an iPhone 12 Pro Max for 1000$, the salesman said that it’s completely new and not activated nor opened. When I searched up the serial number before I activated it said please activate your iPhone. After I activated the phone I put the serial number on the website and it said that the purchase date was on April 1978 which is completely impossible, and my coverage was expired when I just bought the phone. One last question, my phone’s model number starts with 5G, please tell me what that means and why is my coverage expired and why the purchase date is April 1978. Thank you.