Flash OEM ROM to Google Pixel Slate

Hello, 
I have several Google pixel slates that I bought pre-owned. They work perfectly, except for a message/alert upon initial set up, about a malformed hardware ID. From what I’ve read, it’s a possibility that someone rooted/flashed a custom ROM and lost the original hardware ID in the process.
mrchromebox.com is a source I’ve looked at. But honestly, I don’t understand how to go about it. 

Looking for some possible help or directions. Much appreciated.